Make use of precompiled header configureable

[coldtobi]

This pull request makes the use of the precompiled header configureable.

Default is kept -- when not tweaking the settings the precompiled headers are used.

Reason behind:
For packaging I rebuild often, and when precompiled headers are active I cannot use tools like ccache to accelerate building. (Also precompiled headers do not really bring compilation speed)

Here some metrics (building the Debian package, using debuild -j4)

normalbuild: (without using ccache and using precompiled headers)
real 8m47.637s

with patch (cold ccache)
real 9m19.478s

hot ccache (as previous, but re-run with the now initialized ccache)
real 1m3.417s

tobi

[RobertBeckebans]

Yeah I also got the impression that precompiled headers are only a big win for MSVC