Enable compiling Ruby 1.8 on OS X 10.8+ without extra flags

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`.

Fixes #193 #207

References 353246926c9273329391915a125ff241e9954ba6
Merge branch 'auto-tcltk'