[base-gcc/sparc64] Fix build for x11/mate/control-center

This is just like gnome/control-center. It needs C99.

ok?

--Kurt

Index: Makefile
===================================================================
RCS file: /cvs/ports/x11/mate/control-center/Makefile,v
retrieving revision 1.21
diff -u -p -r1.21 Makefile
--- Makefile 21 Mar 2020 16:14:54 -0000 1.21
+++ Makefile 28 Mar 2020 05:01:57 -0000
@@ -46,4 +46,6 @@ FAKE_FLAGS= sysconfdir=${PREFIX}/share/

CONFIGURE_STYLE= gnu

+CFLAGS += -std=gnu99
+
.include <bsd.port.mk>