Replace constants.h with buildsystem defines

@BugFreeSoftware, please test this one too.
I though you'd maybe need the same defines if you'd compile on Windows without MSVC.

I just asked in the #cmake channel on freenode, WIN32 triggers on every Windows, 32 and 64bit:

[19:53] <lclc> does a if(WIN32) detect 32 and 64 bit Windows?
[19:53] <ngladitz> it is set for both, yes