ffbuild/common: Fix CPPFLAGS applied for compiling C++ files

Currently, $(CPPFLAGS) and $(CFLAGS) are prepended to CXXFLAGS
(the flags for compiling C++) like this:
CXXFLAGS := $(CPPFLAGS) $(CFLAGS) $(CXXFLAGS)
Using ":=" creates a simply expanded variable, i.e. the values
of the variable at the time of assignment are used and later
modifications to them are ignored (using a recursively expanding
variable (i.e. "=" instead of ":=") is not really possible here,
as there would be an infinite loop when evaluating CXXFLAGS).

Yet we perform later additions to CPPFLAGS: HAVE_AV_CONFIG_H and
BUILDING_libfoo are defined. These do not reach C++ compilations.

To fix this a trick is employed to prepend to a recursively
expanded variable while keeping it recursively expanded.

There are two practical consequences of this: C++ files now no longer
include the version.h header, but only the version_major.h header
of their library, saving some recompilations. Furthermore, they
now get some optimized math functions (namely the ones from
lavu/intmath.h instead of the ones from lavu/common.h).
(av_parity() is the only one for which it makes a difference.)

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

ffbuild/common.mak

@@ -29,6 +29,9 @@ $(foreach VAR,$(SILENT),$(eval override $(VAR) = @$($(VAR))))
 $(eval INSTALL = @$(call ECHO,INSTALL,$$(^:$(SRC_DIR)/%=%)); $(INSTALL))
 endif
 
+# Prepend to a recursively expanded variable without making it simply expanded.
+PREPEND = $(eval $(1) = $(patsubst %,$$(%), $(2)) $(value $(1)))
+
 # NASM requires -I path terminated with /
 IFLAGS     := -I. -I$(SRC_LINK)/
 CPPFLAGS   := $(IFLAGS) $(CPPFLAGS)
@@ -37,7 +40,9 @@ CCFLAGS     = $(CPPFLAGS) $(CFLAGS)
 OBJCFLAGS  += $(EOBJCFLAGS)
 OBJCCFLAGS  = $(CPPFLAGS) $(CFLAGS) $(OBJCFLAGS)
 ASFLAGS    := $(CPPFLAGS) $(ASFLAGS)
-CXXFLAGS   := $(CPPFLAGS) $(CFLAGS) $(CXXFLAGS)
+# Use PREPEND here so that later (target-dependent) additions to CPPFLAGS
+# end up in CXXFLAGS.
+$(call PREPEND,CXXFLAGS, CPPFLAGS CFLAGS)
 X86ASMFLAGS += $(IFLAGS:%=%/) -I$(<D)/ -Pconfig.asm
 
 HOSTCCFLAGS = $(IFLAGS) $(HOSTCPPFLAGS) $(HOSTCFLAGS)