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).
CXXFLAGS = @CXXFLAGS@
LIBCXXFLAGS = $(CXXFLAGS) -fno-implicit-templates
GOCFLAGS = $(CFLAGS)
-GDCFLAGS = $(CFLAGS)
+GDCFLAGS = @GDCFLAGS@
GM2FLAGS = $(CFLAGS)
PKG_CONFIG_PATH = @PKG_CONFIG_PATH@