Native Windows support with MinGW

The game can now be compiled with the MinGW toolchain, creating a native
Windows version of the game that requires no other dependencies to run.
At last, proper Windows support has been restored!

The rough steps (proper instructions for the readme to come):

 1. Install MinGW.
 2. Right click "Computer" -> "Advanced Settings" ->
    "Environment Variables".
 3. Add new PATH variable, set it to "C:\MinGW\bin".
 4. Console: mingw-get install msys-flex-bin msys-bison-bin mingw-libz
 5. Download and extract the latest Jansson off GitHub.
 6. Edit out -fPIC from Jansson's top-level CMakeLists.txt.
 7. Launch CMake, point at Jansson, "Configure" and "Generate".
 8. Build and install Jansson with 'mingw32-make install'.
 9. Download and extract PDCurses.
 10. In PDCurses' win32 folder: mingw32-make -f mingwin32.mak WIDE=Y
 11. Rename "pdcurses.a" to "libpdcurses.a".
 12. Download and extract this game's source code.
 13. Launch CMake, point it to this game's source code.
 14. Set BISON_EXECUTABLE to "C:\MinGW\msys\1.0\bin\bison.exe"
     and FLEX_EXECUTABLE to "C:\MinGW\msys\1.0\bin\flex.exe".
 15. Set Jansson and PDCurses include and lib paths.
 16. Set final install path for the game (same path for all is fine).
 17. Click "Configure" and "Generate".
 18. In the game's build folder: mingw32-make install
 19. The game can be found in the install path, ready to play/zip up.

MinGW is a fraction of the size of Cygwin (200~ MB vs. 500~ MB), but the
game standalone is just 3 MB.

Known issue: ALL_STATIC option fails to link the final EXE properly,
since its link line doesn't include the lib path for Jansson.