Add XQuartz's path for X11

Because it seems that /usr/X11 has been unused since OS X 10.7.
XQuartz (/opt/X11) is a substitute for /usr/X11.
Merge pull request #10 from myokoym/add-xquartz-path-for-x11

Add XQuartz's path for X11

Patch by Masafumi Yokoyama. Thanks!!!