Mac OS X 10.6 support

Changes from vit9696.  For notes (especially the `!defined()` thing not working), see here:

http://forums.ppsspp.org/showthread.php?tid=1826&pid=12402#pid12402

Tested on 10.6 and 10.8, expected to work on 10.7  Also includes a change to no longer look in the user directory for assets/.

-[Unknown]