Make sure to check for glu.h before enabling glX support. We don't

 directly need GLU, but SDL_opengl.h tries to include it, and it shows that
 the workstation is misconfigured or incompletely configured if just this
 one header is missing.

   Fixes Bugzilla #336.

--HG--
branch : SDL-1.2
extra : convert_revision : svn%3Ac70aab31-4412-0410-b14c-859654838e24/branches/SDL-1.2%402358

configure.in

@@ -1550,6 +1550,7 @@ CheckOpenGLX11()
         AC_TRY_COMPILE([
          #include <GL/gl.h>
          #include <GL/glx.h>
+         #include <GL/glu.h>
         ],[
         ],[
         video_opengl=yes