Support using the homebrewed installation of Python with GDB

Adds a flag, --with-brewed-python, which will build GDB with the
Homebrewed Python installation instead of the system one. This requires
Python to be built with --universal so a check for this has been added
as well.

Closes #153.

Signed-off-by: Xiyue Deng <manphiz@gmail.com>
Style nitpick for args.