[CMake] Support for MacRuby?

[Dave Abrahams]

Macruby (http://macruby.org)'s installer is a bit unusual:

It will install MacRuby in /Library/Frameworks and provide shortcuts
to its executable utilities in /usr/local/bin, with a "mac" prefix.  For
example, "ruby" will be available as "macruby", and "irb" as  "macirb".

It would be nice if the find_package(ruby) command were able to find
and use MacRuby if that is the newest Ruby installation.