Draft: JIT functors using precompiled JIT_includes.h instead of preprocessed header

Replaces the 'preprocessed header' stuff with a Precompiled Header installed next to JIT_includes.h. This header should be automatically picked up by the compiler when jitting functors.

The idea is to see if performance and RAM usage is improved at configure time which is going to help with DaVinci#175

I think that it might help, because functor JITting at the moment basically involves compiling some functor code with a large header file each time, which a precompiled header might help with.

This is not ready to use as I have not been able to test it properly yet - don't know if it will actually work or not.

cc @cburr