[sparc64/base-gcc] Fix build of audio/mpg123

The most recent version of mpg123 uses C99 for loop initialization.

This fixes the build for sparc64/base-gcc.

ok?

(cc maintainer)

--Kurt

Index: Makefile
===================================================================
RCS file: /cvs/ports/audio/mpg123/Makefile,v
retrieving revision 1.107
diff -u -p -r1.107 Makefile
--- Makefile 11 Jun 2021 20:55:07 -0000 1.107
+++ Makefile 12 Jun 2021 02:24:18 -0000
@@ -36,4 +36,9 @@ pre-patch:
N; s/\$$(LIBTOOLFLAGS)/& --tag=disable-static/; \
}' Makefile.in.beforesubst >Makefile.in

+.include <bsd.port.arch.mk>
+.if !${PROPERTIES:Mclang}
+CFLAGS += -std=gnu99
+.endif
+
.include <bsd.port.mk>