Find X11 library and header to fix tkinter compile error. Fixes #33

With version of MacOSX > 11.x (Lion), Apple has dropped dedicated support
for X11.app, with users directed to the open source XQuartz project
instead. See http://en.wikipedia.org/wiki/XQuartz

Since headers and libraries provided by XQuartz are located in the
non system location '/opt/X11', it is required to find them.