Linux SDL gamma bug workaround using XF86

I didn't see a practical way to work around this issue without bringing in extra deps (I considered drawing a rectangle over the screen with the correct opacity), but this seemed like the simplest way. It uses the same function (XF86VidModeSetGamma) that's used in the non-SDL version (found in linux_glimp.c). I've tested this, and it is working on my system.

This fix brings in dependencies for libX11.so and libXxf86vm.so.