Try setuptools first, then fallback to distutils.core

Hi,

This is a request related to the OpenBSD port for youtube-dl.

I'd like this diff to be applied because our ports framework uses 'install --single-version-externally-managed' when dealing with setuptools-based python packages.

Currently, packaging of youtube-dl fails with the error:

===>  Faking installation for youtube-dl-2013.01.28
usage: setup.py [global_opts] cmd1 [cmd1_opts] [cmd2 [cmd2_opts] ...]
   or: setup.py --help [cmd1 cmd2 ...]
   or: setup.py --help-commands
   or: setup.py cmd --help

error: option --single-version-externally-managed not recognized

*** Error 1 in . (/usr/ports/lang/python/python.port.mk:181 'do-install': @cd /usr/ports/pobj/youtube-dl-2013.01.28/youtube-dl && /usr/bin/e...)
*** Error 1 in . (/usr/ports/infrastructure/mk/bsd.port.mk:2678 '/usr/ports/pobj/youtube-dl-2013.01.28/fake-amd64/.fake_done')
*** Error 1 in /usr/ports/mystuff/www/youtube-dl (/usr/ports/infrastructure/mk/bsd.port.mk:2323 'fake')

Please consider merging my almost unobtrusive change.