games/cave9: fix build on GCC architectures

In file included from display.c:19:
/usr/local/include/SDL/SDL_opengl.h:116:1: error: "GL_GLEXT_VERSION" redefined
In file included from /usr/local/include/GL/gl.h:2050,

from /usr/local/include/SDL/SDL_opengl.h:46,
from display.c:19:

/usr/local/include/GL/glext.h:54:1: error: this is the location of the previous definition