Because OS X Mountain Lion removed X Windows, compiling Ruby 1.8 would
fail unless the user installed XQuartz manually and passed:
CPPFLAGS=-I/opt/X11/include rbenv install 1.8.7-p374
This auto-detects if `/opt/X11/include` is present on the system and
configures CPPFLAGS accordingly. However if XQuartz was never installed,
we simply configure Ruby using `--without-tk`.