Add build support for OSX 10.9 and make Perforce optional

This PR adds support for compiling on OS X 10.9.

It also makes compiling with Perforce on Linux/OSX optional. This is because lots of people don't have the exact Perforce setup required for those commands to work – or don't want to worry setting up that sort of a server when they just use Git for their version control.

By default, it'll execute the Perforce commands as normal. However, if you call make with the `PERFORCE ` environment variable set to `false`, it'll ignore those commands. Makes everything nicer because you don't need to see a bunch of unnecessary Perforce client errors every compile.

eg: ```PERFORCE=false make```

If you don't want the Perforce changes and just want the OSX 10.9 support, let me know and I can just split out the 10.9 support to its own PR.