Support building the X11 dissector outside the source tree.

Support building the X11 dissector outside the source tree.

Use relative path names for the script names and mesa/xcbproto directories so
different paths (from different users) don't cause deltas.

Also, with help from Peter Wu and Graham, get generation of the X11 dissector
working from cmake.

Change-Id: I95cd2a0f973dcbc67f457ff94c28e46a666afb74
Reviewed-on: https://code.wireshark.org/review/12213
Petri-Dish: Jeff Morriss <jeff.morriss.ws@gmail.com>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Alexis La Goutte <alexis.lagoutte@gmail.com>
Reviewed-by: Peter Wu <peter@lekensteyn.nl>