Change the way SDL.h is included to standard cross-platform way.

Fallgergeist is currently intended to be run on unix-like systems only, but SDL is a cross platform library and it supports a lot of systems Windows included. The most portable way to include SDL.h is #include "SDL.h" ( http://wiki.libsdl.org/FAQDevelopment#Do_I_.23include_.3CSDL.h.3E_or_.3CSDL.2FSDL.h.3E.3F ) .
Cmake script had to be corrected to add directory with SDL.h to include path.

P.S.
I tried to compile falltergeist under Windows and after removing some linux specific code (handling files) managed 
to succeed. Falltergeist doesn't work properly, because of slashes inside paths I think, but it is no big deal.