Enable MinGW (Windows) build

* Correct an error in the SDL include directory (possibly need to use
both types. From CMake 2.8 the FindSDL.cmake module uses
SDL_INCLUDE_DIR)
* Set the executable to Windows GUI app rather than console app
* Add the -posix flag to MinGW in order to support full C99 printf
formatters (such as %z)
Missing changes to common.h

* Search for SDL directly because SDL include directory is now correctly
referenced by CMake
Merge pull request #3 from BrianSidebotham/master

Enable MinGW (Windows) build