Support Apple X11 at /opt/X11

[Daniel Macks]

Apple has migrated its X11 suite (xquartz) to /opt/X11 for the past few OS X 
major releases, but they have retained some symlinks from /usr/X11 and/or 
/usr/X11R6 so that we can still access headers and libs via -I/usr/X11/include 
and similar. A few years ago, they temporarily dropped a symlink, and many 
things became uncompileable unless one knew to pass --x-includes= and 
--x-libraries paths. Rumor is they might be doing it again in the next release. 
If I patch libs.m4 to place /opt/X11/include in the ac_x_header_dirs default 
search paths list, X11 is detected correctly. 

dan

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Daniel Macks
address@hidden