Generate MSI installer for Windows.

Setup:

    apt-get install wixl

Build (using sok.msi build target):

    ninja sok.msi

Dependencies:

* `SDL2-devel` (from libsdl.org)
* `SDL2_ttf-devel` (from libsdl.org)

Example (generate by hand):
    
    # generate sok.msi
    wixl -o sok.msi \
      -D BUILD_DIR=../../build-w64 \
      -D SDL_LIB_DIR=/usr/local/x86_64-w64-mingw32/bin \
      sok.wxs