Makefiles: don't use -xobjective-c when compiling C++ files on Mac

On Mac, pass -xobjective-c to the compiler through AM_CFLAGS, not
AM_CPPFLAGS, so that it's only used for C sources, and not C++
sources.  In the latter case, it clashes with the -std=... flag,
spewing an error.  Thanks, Partha :)

libgimpbase/Makefile.am

@@ -64,7 +64,9 @@ AM_CPPFLAGS = \
 	$(GIO_CFLAGS)						\
 	$(GEXIV2_CFLAGS)					\
 	$(BINRELOC_CFLAGS)					\
-	-I$(includedir)						\
+	-I$(includedir)
+
+AM_CFLAGS = \
 	$(xobjective_c)
 
 AM_CCASFLAGS = \