Don't use deprecated symbol forms of Autotools
Make GUI build optional, only allow on Mavericks
Don't require Mavericks for GUI builds

I previously thought the GUI was causing a build failure, but that
doesn’t seem to be the case.
Fix boost location

I erroneously referenced boost’s include path in my last commit when we
just want the prefix.
Don't use deprecated symbol forms of Autotools
Use a 'stable do' block