Teach perl/Tk where OS X Yosemite Xquartz puts the X11 files

They now go in /opt/X11 and the system /usr/ tree no longer has a
residual soft link to that location (as was the case with Mavericks).
Merge pull request #12 from timj/yosemite-x11

Teach perl/Tk where OS X Yosemite Xquartz puts the X11 files