OS X: Fix build on Mac OS X 10.6 (from vit9696.)
OS X: Look for assets/ in prog dir (from vit9696.)
OS X: Workaround weird problem in OS X 10.6 with defines.

Apparently !defined(__MAC_10_6) doesn't work?
Merge pull request #1163 from unknownbrackets/mac10.6

Mac OS X 10.6 support