toplevel: Substitute GDCFLAGS instead of using CFLAGS

r14-2875-g1ed21e23d6d4da ("Use substituted GDCFLAGS") already
implemented this change, but only on the generated file rather than in
the template it is generated from.

	* Makefile.tpl: Substitute @GDCFLAGS@ instead of using
	$(CFLAGS).

Makefile.tpl

@@ -448,7 +448,7 @@ LIBCFLAGS = $(CFLAGS)
 CXXFLAGS = @CXXFLAGS@
 LIBCXXFLAGS = $(CXXFLAGS) -fno-implicit-templates
 GOCFLAGS = $(CFLAGS)
-GDCFLAGS = $(CFLAGS)
+GDCFLAGS = @GDCFLAGS@
 GM2FLAGS = $(CFLAGS)
 
 PKG_CONFIG_PATH = @PKG_CONFIG_PATH@