Believe it or not, it is possible to have Linux without GLX.

This patch sets PIGLIT_HAS_GLX on Linux only if X11 and libGL is found.
Otherwise, an EGL-only Linux system would fail to build
piglit_x11_framework, since it could not find <GL/glx.h>.

v1: djkurtz
v2: chadv
    - The GLX protocol headers (glproto.pc) are required to build the
      GLX tests but are not required to build OpenGL tests that use
      simply use (rather than test) GLX.  So remove dependency of
      PIGLIT_HAS_GLX on glproto.

Signed-off-by: Daniel Kurtz <djkurtz@chromium.org>
Signed-off-by: Chad Versace <chad.versace@linux.intel.com>