I just commited 6e343e0655d8, which introduces support for gcc's precompiled header implementation. It can cut the build time in a half. Although some sources declared that use of CMake experimental I considered it safe enough to leave it default turned on. Turning USE_GCC_PCH Off will disable it.

Note: If you are using ccache to speed up your build, you'll not benefit from that patch unless you set CCACHE_SLOPPINESS=time_macros. See the ccache docs for more.

Some data acquired with:
make clean rebuild_cache
time make clonk c4group

Cross win32:
real    4m21.620s
user    3m45.326s
sys     0m26.806s

real    2m17.858s
user    1m51.411s
sys     0m16.357s

Native linux64 dedi:
real    5m17.464s
user    4m34.005s
sys     0m28.042s

real    2m49.237s
user    2m14.052s
sys     0m17.133s

I hope the wicked tweaks were worth it...