No OpenGL Video Output if XCB disabled

VLC fails to find any "gl" display modules if built with xcb disabled (--disable-xcb) and opengl enabled (--enable-glx & --enable-glspectrum). This was tested with vlc-2.2.1. The same video plays fine with xcb is enabled.

Sample video used for test: http://techslides.com/demos/sample-videos/small.mp4

Debug logs of attempting to play the test video with the OpenGL video output when VLC is built with xcb disabled & xcb enabled are attached.

Downstream bug report: https://bugs.gentoo.org/show_bug.cgi?id=539840

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