glx: More hacking around versions of XCB that lack GLX_ARB_create_context support

glx: More hacking around versions of XCB that lack GLX_ARB_create_context support

Detect whether a new enough version of XCB is installed at configure
time.  If it is not, don't enable the extension and don't build the
unit tests.

v2: Move the AM_CONDIATION outside the case-statement so that it is
invoked even for non-GLX builds.  This prevents build failures with
osmesa, for example.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Tested-by: Robert Hooker <robert.hooker@canonical.com>