Fixed cross compilations with the Autotools build

Pull request #2517 caused cross compilations to start failing, because
the js_embed binary was being built to run on the target platform
instead of on the build machine. This change updates the Autotools build
to use the AX_PROG_CXX_FOR_BUILD macro to find a suitable compiler for
the build machine and always use that when building js_embed.

configure.ac

@@ -60,6 +60,7 @@ AC_ARG_WITH([protoc],
 # Checks for programs.
 AC_PROG_CC
 AC_PROG_CXX
+AC_PROG_CXX_FOR_BUILD
 AC_LANG([C++])
 ACX_USE_SYSTEM_EXTENSIONS
 m4_ifdef([AM_PROG_AR], [AM_PROG_AR])

m4/ax_prog_cc_for_build.m4

@@ -0,0 +1,125 @@
+# ===========================================================================
+#   http://www.gnu.org/software/autoconf-archive/ax_prog_cc_for_build.html
+# ===========================================================================
+#
+# SYNOPSIS
+#
+#   AX_PROG_CC_FOR_BUILD
+#
+# DESCRIPTION
+#
+#   This macro searches for a C compiler that generates native executables,
+#   that is a C compiler that surely is not a cross-compiler. This can be
+#   useful if you have to generate source code at compile-time like for
+#   example GCC does.
+#
+#   The macro sets the CC_FOR_BUILD and CPP_FOR_BUILD macros to anything
+#   needed to compile or link (CC_FOR_BUILD) and preprocess (CPP_FOR_BUILD).
+#   The value of these variables can be overridden by the user by specifying
+#   a compiler with an environment variable (like you do for standard CC).
+#
+#   It also sets BUILD_EXEEXT and BUILD_OBJEXT to the executable and object
+#   file extensions for the build platform, and GCC_FOR_BUILD to `yes' if
+#   the compiler we found is GCC. All these variables but GCC_FOR_BUILD are
+#   substituted in the Makefile.
+#
+# LICENSE
+#
+#   Copyright (c) 2008 Paolo Bonzini <bonzini@gnu.org>
+#
+#   Copying and distribution of this file, with or without modification, are
+#   permitted in any medium without royalty provided the copyright notice
+#   and this notice are preserved. This file is offered as-is, without any
+#   warranty.
+
+#serial 8
+
+AU_ALIAS([AC_PROG_CC_FOR_BUILD], [AX_PROG_CC_FOR_BUILD])
+AC_DEFUN([AX_PROG_CC_FOR_BUILD], [dnl
+AC_REQUIRE([AC_PROG_CC])dnl
+AC_REQUIRE([AC_PROG_CPP])dnl
+AC_REQUIRE([AC_EXEEXT])dnl
+AC_REQUIRE([AC_CANONICAL_HOST])dnl
+
+dnl Use the standard macros, but make them use other variable names
+dnl
+pushdef([ac_cv_prog_CPP], ac_cv_build_prog_CPP)dnl
+pushdef([ac_cv_prog_gcc], ac_cv_build_prog_gcc)dnl
+pushdef([ac_cv_prog_cc_works], ac_cv_build_prog_cc_works)dnl
+pushdef([ac_cv_prog_cc_cross], ac_cv_build_prog_cc_cross)dnl
+pushdef([ac_cv_prog_cc_g], ac_cv_build_prog_cc_g)dnl
+pushdef([ac_cv_exeext], ac_cv_build_exeext)dnl
+pushdef([ac_cv_objext], ac_cv_build_objext)dnl
+pushdef([ac_exeext], ac_build_exeext)dnl
+pushdef([ac_objext], ac_build_objext)dnl
+pushdef([CC], CC_FOR_BUILD)dnl
+pushdef([CPP], CPP_FOR_BUILD)dnl
+pushdef([CFLAGS], CFLAGS_FOR_BUILD)dnl
+pushdef([CPPFLAGS], CPPFLAGS_FOR_BUILD)dnl
+pushdef([LDFLAGS], LDFLAGS_FOR_BUILD)dnl
+pushdef([host], build)dnl
+pushdef([host_alias], build_alias)dnl
+pushdef([host_cpu], build_cpu)dnl
+pushdef([host_vendor], build_vendor)dnl
+pushdef([host_os], build_os)dnl
+pushdef([ac_cv_host], ac_cv_build)dnl
+pushdef([ac_cv_host_alias], ac_cv_build_alias)dnl
+pushdef([ac_cv_host_cpu], ac_cv_build_cpu)dnl
+pushdef([ac_cv_host_vendor], ac_cv_build_vendor)dnl
+pushdef([ac_cv_host_os], ac_cv_build_os)dnl
+pushdef([ac_cpp], ac_build_cpp)dnl
+pushdef([ac_compile], ac_build_compile)dnl
+pushdef([ac_link], ac_build_link)dnl
+
+save_cross_compiling=$cross_compiling
+save_ac_tool_prefix=$ac_tool_prefix
+cross_compiling=no
+ac_tool_prefix=
+
+AC_PROG_CC
+AC_PROG_CPP
+AC_EXEEXT
+
+ac_tool_prefix=$save_ac_tool_prefix
+cross_compiling=$save_cross_compiling
+
+dnl Restore the old definitions
+dnl
+popdef([ac_link])dnl
+popdef([ac_compile])dnl
+popdef([ac_cpp])dnl
+popdef([ac_cv_host_os])dnl
+popdef([ac_cv_host_vendor])dnl
+popdef([ac_cv_host_cpu])dnl
+popdef([ac_cv_host_alias])dnl
+popdef([ac_cv_host])dnl
+popdef([host_os])dnl
+popdef([host_vendor])dnl
+popdef([host_cpu])dnl
+popdef([host_alias])dnl
+popdef([host])dnl
+popdef([LDFLAGS])dnl
+popdef([CPPFLAGS])dnl
+popdef([CFLAGS])dnl
+popdef([CPP])dnl
+popdef([CC])dnl
+popdef([ac_objext])dnl
+popdef([ac_exeext])dnl
+popdef([ac_cv_objext])dnl
+popdef([ac_cv_exeext])dnl
+popdef([ac_cv_prog_cc_g])dnl
+popdef([ac_cv_prog_cc_cross])dnl
+popdef([ac_cv_prog_cc_works])dnl
+popdef([ac_cv_prog_gcc])dnl
+popdef([ac_cv_prog_CPP])dnl
+
+dnl Finally, set Makefile variables
+dnl
+BUILD_EXEEXT=$ac_build_exeext
+BUILD_OBJEXT=$ac_build_objext
+AC_SUBST(BUILD_EXEEXT)dnl
+AC_SUBST(BUILD_OBJEXT)dnl
+AC_SUBST([CFLAGS_FOR_BUILD])dnl
+AC_SUBST([CPPFLAGS_FOR_BUILD])dnl
+AC_SUBST([LDFLAGS_FOR_BUILD])dnl
+])

m4/ax_prog_cxx_for_build.m4

@@ -0,0 +1,110 @@
+# ===========================================================================
+#   http://www.gnu.org/software/autoconf-archive/ax_prog_cxx_for_build.html
+# ===========================================================================
+#
+# SYNOPSIS
+#
+#   AX_PROG_CXX_FOR_BUILD
+#
+# DESCRIPTION
+#
+#   This macro searches for a C++ compiler that generates native
+#   executables, that is a C++ compiler that surely is not a cross-compiler.
+#   This can be useful if you have to generate source code at compile-time
+#   like for example GCC does.
+#
+#   The macro sets the CXX_FOR_BUILD and CXXCPP_FOR_BUILD macros to anything
+#   needed to compile or link (CXX_FOR_BUILD) and preprocess
+#   (CXXCPP_FOR_BUILD). The value of these variables can be overridden by
+#   the user by specifying a compiler with an environment variable (like you
+#   do for standard CXX).
+#
+# LICENSE
+#
+#   Copyright (c) 2008 Paolo Bonzini <bonzini@gnu.org>
+#   Copyright (c) 2012 Avionic Design GmbH
+#
+#   Based on the AX_PROG_CC_FOR_BUILD macro by Paolo Bonzini.
+#
+#   Copying and distribution of this file, with or without modification, are
+#   permitted in any medium without royalty provided the copyright notice
+#   and this notice are preserved. This file is offered as-is, without any
+#   warranty.
+
+#serial 2
+
+AU_ALIAS([AC_PROG_CXX_FOR_BUILD], [AX_PROG_CXX_FOR_BUILD])
+AC_DEFUN([AX_PROG_CXX_FOR_BUILD], [dnl
+AC_REQUIRE([AX_PROG_CC_FOR_BUILD])dnl
+AC_REQUIRE([AC_PROG_CXX])dnl
+AC_REQUIRE([AC_PROG_CXXCPP])dnl
+AC_REQUIRE([AC_CANONICAL_HOST])dnl
+
+dnl Use the standard macros, but make them use other variable names
+dnl
+pushdef([ac_cv_prog_CXXCPP], ac_cv_build_prog_CXXCPP)dnl
+pushdef([ac_cv_prog_gxx], ac_cv_build_prog_gxx)dnl
+pushdef([ac_cv_prog_cxx_works], ac_cv_build_prog_cxx_works)dnl
+pushdef([ac_cv_prog_cxx_cross], ac_cv_build_prog_cxx_cross)dnl
+pushdef([ac_cv_prog_cxx_g], ac_cv_build_prog_cxx_g)dnl
+pushdef([CXX], CXX_FOR_BUILD)dnl
+pushdef([CXXCPP], CXXCPP_FOR_BUILD)dnl
+pushdef([CXXFLAGS], CXXFLAGS_FOR_BUILD)dnl
+pushdef([CPPFLAGS], CPPFLAGS_FOR_BUILD)dnl
+pushdef([CXXCPPFLAGS], CXXCPPFLAGS_FOR_BUILD)dnl
+pushdef([host], build)dnl
+pushdef([host_alias], build_alias)dnl
+pushdef([host_cpu], build_cpu)dnl
+pushdef([host_vendor], build_vendor)dnl
+pushdef([host_os], build_os)dnl
+pushdef([ac_cv_host], ac_cv_build)dnl
+pushdef([ac_cv_host_alias], ac_cv_build_alias)dnl
+pushdef([ac_cv_host_cpu], ac_cv_build_cpu)dnl
+pushdef([ac_cv_host_vendor], ac_cv_build_vendor)dnl
+pushdef([ac_cv_host_os], ac_cv_build_os)dnl
+pushdef([ac_cxxcpp], ac_build_cxxcpp)dnl
+pushdef([ac_compile], ac_build_compile)dnl
+pushdef([ac_link], ac_build_link)dnl
+
+save_cross_compiling=$cross_compiling
+save_ac_tool_prefix=$ac_tool_prefix
+cross_compiling=no
+ac_tool_prefix=
+
+AC_PROG_CXX
+AC_PROG_CXXCPP
+
+ac_tool_prefix=$save_ac_tool_prefix
+cross_compiling=$save_cross_compiling
+
+dnl Restore the old definitions
+dnl
+popdef([ac_link])dnl
+popdef([ac_compile])dnl
+popdef([ac_cxxcpp])dnl
+popdef([ac_cv_host_os])dnl
+popdef([ac_cv_host_vendor])dnl
+popdef([ac_cv_host_cpu])dnl
+popdef([ac_cv_host_alias])dnl
+popdef([ac_cv_host])dnl
+popdef([host_os])dnl
+popdef([host_vendor])dnl
+popdef([host_cpu])dnl
+popdef([host_alias])dnl
+popdef([host])dnl
+popdef([CXXCPPFLAGS])dnl
+popdef([CPPFLAGS])dnl
+popdef([CXXFLAGS])dnl
+popdef([CXXCPP])dnl
+popdef([CXX])dnl
+popdef([ac_cv_prog_cxx_g])dnl
+popdef([ac_cv_prog_cxx_cross])dnl
+popdef([ac_cv_prog_cxx_works])dnl
+popdef([ac_cv_prog_gxx])dnl
+popdef([ac_cv_prog_CXXCPP])dnl
+
+dnl Finally, set Makefile variables
+dnl
+AC_SUBST([CXXFLAGS_FOR_BUILD])dnl
+AC_SUBST([CXXCPPFLAGS_FOR_BUILD])dnl
+])

src/Makefile.am

@@ -484,13 +484,16 @@ libprotoc_la_SOURCES =                                         \
   google/protobuf/compiler/csharp/csharp_wrapper_field.cc      \
   google/protobuf/compiler/csharp/csharp_wrapper_field.h
 
-bin_PROGRAMS = protoc js_embed
+bin_PROGRAMS = protoc
 protoc_LDADD = $(PTHREAD_LIBS) libprotobuf.la libprotoc.la
 protoc_SOURCES = google/protobuf/compiler/main.cc
 
 # The special JS code for the well-known types is linked into the compiler via
-# well_known_types_embed.cc, which is generated from .js source files.
+# well_known_types_embed.cc, which is generated from .js source files. We have
-js_embed_SOURCES = google/protobuf/compiler/js/embed.cc
+# to build the js_embed binary using $(CXX_FOR_BUILD) so that it is executable
+# on the build machine in a cross-compilation setup.
+js_embed$(EXEEXT): google/protobuf/compiler/js/embed.cc
+	$(CXX_FOR_BUILD) -o $@ $<
 js_well_known_types_sources =                                  \
 	google/protobuf/compiler/js/well_known_types/any.js          \
 	google/protobuf/compiler/js/well_known_types/struct.js       \