build: reduce GLX test fragment

While it doesn't matter for anything, we don't really need to check for
GL symbols (neither headers nor for linking).

Author: wm4

waftools/fragments/gl_x11.c

@@ -1,15 +1,11 @@
 #include <X11/Xlib.h>
 #include <GL/glx.h>
-#include <GL/gl.h>
 #include <stddef.h>
 
 int main(int argc, char *argv[]) {
   glXCreateContext(NULL, NULL, NULL, True);
   glXQueryExtensionsString(NULL, 0);
   glXGetProcAddressARB("");
   glXGetCurrentDisplay();
-  glFinish();
-  (void)GL_RGB32F;          // arbitrary OpenGL 3.0 symbol
-  (void)GL_LUMINANCE16;     // arbitrary OpenGL legacy-only symbol
   return 0;
 }