setup.py automatically detects Mac OS X and Linux (solves part of GitHub issue #1)

Juan A. Diaz sent me this patch by email. It allows to build the extension on Mac OS X and
Debian without manual modification of setup.py. I tested this on Mac OS X 10.8.2 and Debian
testing (pre-wheezy).

To completely solve issue #1, I require that the build also works on Windows, and possibly
detects other versions of libaprutil.
find built extension for Python 3.2

on Debian testing (pre-wheezy), building the extension under Python 3.2 results in a file
aprmd5.cpython-32mu.so (not plain aprmd5.so)