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

x11/gnome/control-center uses C99 constructs so needs -std=gnu99 for
base-gcc architectures.

ok?

--Kurt

Index: Makefile
===================================================================
RCS file: /cvs/ports/x11/gnome/control-center/Makefile,v
retrieving revision 1.59
diff -u -p -r1.59 Makefile
--- Makefile 16 Feb 2020 09:55:42 -0000 1.59
+++ Makefile 28 Mar 2020 02:47:57 -0000
@@ -25,6 +25,8 @@ MODULES= devel/meson \
security/heimdal \
x11/gnome

+CFLAGS= -std=gnu99
+
LIB_DEPENDS= audio/gsound \
audio/pulseaudio \
devel/libgtop2 \