[sparc64/base-gcc] Fix games/xcowsay build

The new version of xcowsay fails on sparc64 (and presumably other base-gcc
architectures) with errors about needing C99.

This adds C99 and fixes the build. I slipped it into the current sparc64
build and it fixed it.

ok?

--Kurt

Index: Makefile
===================================================================
RCS file: /cvs/ports/games/xcowsay/Makefile,v
retrieving revision 1.15
diff -u -p -r1.15 Makefile
--- Makefile 27 Feb 2020 15:18:22 -0000 1.15
+++ Makefile 3 Mar 2020 03:55:54 -0000
@@ -18,5 +18,6 @@ MASTER_SITES = http://www.nickg.me.uk/f
CONFIGURE_STYLE = gnu

LIB_DEPENDS = x11/gtk+3
+CFLAGS += -std=c99

.include <bsd.port.mk>