Some modifications to the build system, that allow us to only use one main.cpp in...

Some modifications to the build system, that allow us to only use one main.cpp in the future.
Of course, we will need a lot of #ifdefs there...
I could not find out how to specify per-target #defines in cmake, so I use different source files
named build_*.cpp that set a #define and #include main.cpp (actually, main_mono.cpp, which is to be renamed
in the next commit).