When building with ports gcc, compile the C parts with -std=gnu11

to match the visibility of max_align_t between C and C++.

GCC 4.9 defaults to C89 and does not accept the "restrict" keyword
by default.  This causes a configure check to add -std=gnu99 to the
compiler flags.  Since max_align_t is from C11, the -std=gnu99 flag
removes it from visibility and gnutls's copy of gnulib then provides
its own replacement definition of max_align_t.  When gnutls builds
its C++ library, the C++ visibility rules are different.  The
max_align_t from stddef.h reappears and collides with the gnulib
replacement.

security/gnutls/Makefile

@@ -1,4 +1,4 @@
-# $OpenBSD: Makefile,v 1.148 2018/11/05 14:57:09 naddy Exp $
+# $OpenBSD: Makefile,v 1.149 2018/11/05 21:29:50 naddy Exp $
 
 COMMENT=		GNU Transport Layer Security library
 
@@ -74,3 +74,8 @@ pre-test:
 		${WRKSRC}/tests/*.c
 
 .include <bsd.port.mk>
+
+# gcc 4.9: match visibility of max_align_t between CC and CXX
+.if ${CHOSEN_COMPILER} == "ports-gcc"
+CFLAGS += -std=gnu11
+.endif