devel/rgbds: Fix build with clang

- yank _POSIX_C_SOURCE because it breaks the build

See also:	gbdev/rgbds#1091
		gbdev/rgbds#1111
Reported by:	danfe, gerald

devel/rgbds/Makefile

@@ -1,6 +1,6 @@
 PORTNAME=	rgbds
 DISTVERSION=	0.6.1
-PORTREVISION=	1
+PORTREVISION=	2
 CATEGORIES=	devel games
 MASTER_SITES=	https://github.com/gbdev/rgbds/releases/download/v${DISTVERSION}/
 
@@ -14,14 +14,13 @@ LICENSE_FILE=	${WRKSRC}/LICENSE
 LIB_DEPENDS=	libpng.so:graphics/png
 
 USES=		bison compiler:c11 dos2unix pkgconfig
-USE_GCC=	yes # clang >10 fails to build, https://github.com/gbdev/rgbds/issues/1091
 MAKE_ARGS=	Q= # verbose builds
 WRKSRC=		${WRKDIR}/rgbds
 
 do-configure:
-# yank _POSIX_C_SOURCE when there is no support for _ISOC11_SOURCE
-	@cd ${WRKSRC}; ${PRINTF} '#include <assert.h>\nint main(){static_assert(1, "");}' | \
-		${CC} -std=gnu11 -D_POSIX_C_SOURCE=200809L -D_ISOC11_SOURCE -xc - 2>/dev/null || \
-		${REINPLACE_CMD} 's,-D_POSIX_C_SOURCE=200809L,,' ${WRKSRC}/Makefile
+# yank _POSIX_C_SOURCE because it breaks the build, see also:
+# https://github.com/gbdev/rgbds/issues/1091
+# https://github.com/gbdev/rgbds/issues/1111
+	${REINPLACE_CMD} 's,-D_POSIX_C_SOURCE=200809L,,' ${WRKSRC}/Makefile
 
 .include <bsd.port.mk>