If anyone here can remember back in the day when Half-Life 2 was coming out you should remember at one point the source code was leaked. I think I found it on a IRC and downloaded it. In that alone was build scripts and conditional syntax for Mac OS X and Linux Version. Granted they most likely have to overhaul that code because Mac OS X and Linux has changed greatly over the years. It just comes to show that Valve never forgot about poor Linux and Mac, they were just waiting for the right time.