Ignore libpng-config under cross-building configuration,

because it will return the flags for the hosting environment.

* builds/unix/configure.raw: Ignore libpng-config when
`cross_compiling' == yes.

ChangeLog

@@ -1,3 +1,11 @@
+2013-07-30  suzuki toshiya  <mpsuzuki@hiroshima-u.ac.jp>
+
+	Ignore libpng-config under cross-building configuration,
+	because it will return the flags for the hosting environment.
+
+	* builds/unix/configure.raw: Ignore libpng-config when
+	`cross_compiling' == yes.
+
 2013-07-30  Behdad Esfahbod  <behdad@google.com>
 
 	Prevent division by zero by a transparent color.

builds/unix/configure.raw

@@ -291,6 +291,10 @@ if test x$with_png != xno; then
     if ! which libpng-config >/dev/null; then
       AC_MSG_ERROR([`libpng-config' not found;
 either set the LIBPNG_CFLAGS and LIBPNG_LDFLAGS environment variables,
+or pass `--without-png' to the `configure' script.])
+    elif test ${cross_compiling} = yes; then
+      AC_MSG_ERROR([`libpng-config' should not be used in cross-building,
+set the LIBPNG_CFLAGS and LIBPNG_LDFLAGS environment,
 or pass `--without-png' to the `configure' script.])
     fi
     LIBPNG_CFLAGS="`libpng-config --cflags`"