PCH support broken with GCC 4.8

[landauf]

PCH (precompiled headers) don't seem to work with GCC 4.8 anymore. In r9685 I had to add an include directory because otherwise it wouldn't even compile with GCC 4.8. But if I compare compilation times with and without PCH in GCC 4.8, it actually takes a few seconds longer with PCH. In prior versions of GCC the speed-up was usually around 15%.

We should fix that soon. PCH support is defined in cmake/tools/PrecompiledHeaderFiles.cmake