build: introduce GCC_CXXFLAGS

The patch "compositor-android: fix build flags" started using GCC_CFLAGS
for C++ files, too. That lead to the following warnings when building a
C++ file:

cc1plus: warning: command line option "-Wstrict-prototypes" is valid for
Ada/C/ObjC but not for C++
cc1plus: warning: command line option "-Wmissing-prototypes" is valid
for Ada/C/ObjC but not for C++

Introduce GCC_CXXFLAGS, similar to GCC_CFLAGS, but for C++, avoiding the
problematic compiler flags.

Signed-off-by: Pekka Paalanen <ppaalanen@gmail.com>

configure.ac

@@ -206,12 +206,14 @@ AM_CONDITIONAL(HAVE_RSVG_CONVERT, test -n "$RSVG_CONVERT")
 
 
 if test "x$GCC" = "xyes"; then
-	GCC_CFLAGS="-Wall -Wextra -Wstrict-prototypes -Wmissing-prototypes \
-		-Wno-unused-parameter -Wno-missing-field-initializers \
-		-g -fvisibility=hidden"
-
+	my_common_gcc_flags="-Wall -Wextra -Wno-unused-parameter \
+		-Wno-missing-field-initializers -g -fvisibility=hidden"
+	GCC_CFLAGS="$my_common_gcc_flags \
+		-Wstrict-prototypes -Wmissing-prototypes"
+	GCC_CXXFLAGS="$my_common_gcc_flags"
 fi
 AC_SUBST(GCC_CFLAGS)
+AC_SUBST(GCC_CXXFLAGS)
 
 WAYLAND_SCANNER_RULES(['$(top_srcdir)/protocol'])
 

src/Makefile.am

@@ -130,7 +130,7 @@ android_backend = android-backend.la
 android_backend_la_LDFLAGS = -module -avoid-version
 android_backend_la_LIBADD = $(COMPOSITOR_LIBS)
 android_backend_la_CFLAGS = $(GCC_CFLAGS) $(COMPOSITOR_CFLAGS)
-android_backend_la_CXXFLAGS = $(GCC_CFLAGS) $(COMPOSITOR_CFLAGS)
+android_backend_la_CXXFLAGS = $(GCC_CXXFLAGS) $(COMPOSITOR_CFLAGS)
 android_backend_la_SOURCES = \
 	compositor-android.c \
 	android-framebuffer.cpp \