Improve recompile speed: Don't include git_rev.h/git_revision.h in system.h

Tested on Windows/Core i7/SSD (to qualify system specs, `make -j8` from scratch on linux x64 takes between 6 and 6.5 minutes):

Before the patch, updating git_rev.h and recompiling (on windows) averages about 16 minutes. With this patch, updating git_rev.h and recompiling averages about 1 minute, 10 seconds, a big improvement.

Needs some Xcode loving, but it's only a new header so it should compile fine until then.