Append the user CPPFLAGS value to the CPPFLAGS used for compilation,

rather than prepend it.

This way there is the guarantee that the included headers are those in
the FFmpeg sources, rather than pre-installed headers referenced by
the user CPPFLAGS.

Originally committed as revision 19416 to svn://svn.ffmpeg.org/ffmpeg/trunk

common.mak

@@ -18,7 +18,7 @@ endif
 
 ALLFFLIBS = avcodec avdevice avfilter avformat avutil postproc swscale
 
-CPPFLAGS += -DHAVE_AV_CONFIG_H -I$(BUILD_ROOT_REL) -I$(SRC_PATH)
+CPPFLAGS := -DHAVE_AV_CONFIG_H -I$(BUILD_ROOT_REL) -I$(SRC_PATH) $(CPPFLAGS)
 
 %.o: %.c
 	$(CC) $(CPPFLAGS) $(CFLAGS) $(LIBOBJFLAGS) -c $(CC_O) $<