fix mingw build on windows

cmake 2.8.11: FindSDL.cmake does not define SDL_INCLUDE_DIRS, only SDL_INCLUDE_DIR. Check it on your system please.

Default path to SDL_ttf.h is include/SDL/SDL_ttf.h. I have wrong installation?

SDLMAIN_LIBRARY is required to build exe on windows.

Tested on win7, mingw 4.8.1 x86