FindGTK2: Move check for pangocairo in gtk module
FindGTK2: Detect gthread library
FindX11: Search in /opt/X11 for OS X 10.8 (#14232)

As of Mac OS X 10.8, X11 is no longer installed under /usr/X11, but
under /opt/X11.
Merge topic 'FindX11-OSX-10.8' into next

488d968 FindX11: Search in /opt/X11 for OS X 10.8 (#14232)
Merge topic 'FindGTK2-tweaks' into next

508e8ca FindGTK2: Detect gthread library
8dca8ce FindGTK2: Move check for pangocairo in gtk module