Rebuilding OptiPNG on Unix systems

This PR rebuilds the OptiPNG project from source on Unix systems if make is installed.  It should leave the existing binaries intact otherwise.

I'm not quite sure how the community deals with a missing make command.  I had to add XCode binaries it to my path myself on OSX 10.8 (did not ship with make).

lmk if I should refactor some things..switch out directory structures, etc. to comply with coding standards.