[sparc64/base-gcc] Fix build of x11/gtk+3

x11/gtk+3 uses C99 constructs, so we need to less base-gcc know that.

ok?

--Kurt

Index: Makefile
===================================================================
RCS file: /cvs/ports/x11/gtk+3/Makefile,v
retrieving revision 1.232
diff -u -r1.232 Makefile
--- Makefile 30 Jun 2020 10:20:47 -0000 1.232
+++ Makefile 3 Jul 2020 06:35:02 -0000
@@ -116,6 +116,11 @@
CONFIGURE_ARGS += --disable-colord
.endif

+.include <bsd.port.arch.mk>
+.if !${PROPERTIES:Mclang}
+CFLAGS += -std=gnu99
+.endif
+
FAKE_FLAGS= sysconfdir=${PREFIX}/share/examples/gtk+3

# configure will disable lpr in default gtk-print-backends if cups is found