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)

NOTE: I tested this on Windows 7 using the mingw-w64 compiler and SDL 1.2.14 from the mingw-w64 project. I tested the emulator by building and running pifox. That works fine. Very impressive work, thanks for the project!!