games/stratagus: fix build on GCC architectures

-Wno-c++11-narrowing is only for clang.

gcc needs -Wno-error:
In file included from /wrkdirs/usr/ports/games/stratagus/work/stratagus-2.4.2/src/include/video.h:46,
                 from /wrkdirs/usr/ports/games/stratagus/work/stratagus-2.4.2/src/action/action_attack.cpp:60:
/usr/local/include/SDL/SDL_opengl.h:116: error: "GL_GLEXT_VERSION" redefined [-Werror]
  116 | #define GL_GLEXT_VERSION 29
      |

games/stratagus/Makefile

@@ -27,8 +27,6 @@ CMAKE_ARGS=	-DGAMEDIR:STRING="bin" \
 		-DTOLUA++_LIBRARY:STRING="${LUA_LIBDIR}/libtolua++.so"
 CMAKE_ON=	ENABLE_DEV
 
-CXXFLAGS+=	-Wno-c++11-narrowing
-
 PORTDOCS=	*
 
 OPTIONS_DEFINE=	MIKMOD MNG THEORA VORBIS X11 FLUIDSYNTH DOCS
@@ -67,4 +65,12 @@ post-install-DOCS-on:
 	@cd ${WRKSRC}/doc && ${COPYTREE_SHARE} "[a-z]*.html *.txt graphics scripts" \
 		${STAGEDIR}${DOCSDIR}
 
-.include <bsd.port.mk>
+.include <bsd.port.pre.mk>
+
+.if ${CHOSEN_COMPILER_TYPE} == clang
+CXXFLAGS+=	-Wno-c++11-narrowing
+.else
+CXXFLAGS+=	-Wno-error
+.endif
+
+.include <bsd.port.post.mk>