games/cube: Unbreak with Clang 6

clients2c.cpp:259:45: error: non-constant-expression cannot be narrowed from type
      'short' to 'float' in initializer list [-Wc++11-narrowing]
            vec v = { ents[i].x, ents[i].y, ents[i].z };
                                            ^~~~~~~~~
clients2c.cpp:352:20: warning: ISO C++11 does not allow conversion from string
      literal to 'char *' [-Wwritable-strings]
            neterr("type");
                   ^

http://beefy12.nyi.freebsd.org/data/head-amd64-default/p475044_s336572/logs/cube-2005.08.29_17.log

- Add missing GL dependency

games/cube/Makefile

@@ -3,7 +3,7 @@
 
 PORTNAME=	cube
 DISTVERSION=	2005_08_29
-PORTREVISION=	17
+PORTREVISION=	18
 CATEGORIES=	games
 MASTER_SITES=	SF/${PORTNAME}/${PORTNAME}/${DISTVERSION}
 DISTFILES=	${CUBE_DATA} ${CUBE_SRC}
@@ -14,6 +14,7 @@ COMMENT=	OpenGL 3D FPS (first person shooter) game
 LIB_DEPENDS=	libenet.so:net/enet
 
 USES=		dos2unix gmake
+USE_CXXSTD=	gnu++98
 EXTRACT_AFTER_ARGS=--exclude enet
 ALL_TARGET=	#
 
@@ -33,7 +34,7 @@ OPTIONS_SUB=	yes
 
 FLAVOR_DESC=	Clients and servers
 CLIENT_DESC=	Build client
-CLIENT_USE=			GL=glu SDL=image,mixer,sdl XORG=x11
+CLIENT_USE=			GL=gl,glu SDL=image,mixer,sdl XORG=x11
 CLIENT_LDFLAGS=			-lX11
 CLIENT_ALL_TARGET=		client
 OPTIMIZED_CFLAGS_CFLAGS=	-O3 -fomit-frame-pointer