Add cross-compilation scripts for Windows

Build the Windows binary from mingw on Linux, using the official
prebuilt binaries for ffmpeg, SDL2 and adb.

MSYS2 and all its packaged dll are not necessary anymore.

cross_win32.txt

@@ -0,0 +1,20 @@
+# apt install mingw-w64 mingw-w64-tools
+
+[binaries]
+name = 'mingw'
+c = '/usr/bin/i686-w64-mingw32-gcc'
+cpp = '/usr/bin/i686-w64-mingw32-g++'
+ar = '/usr/bin/i686-w64-mingw32-ar'
+strip = '/usr/bin/i686-w64-mingw32-strip'
+pkgconfig = '/usr/bin/i686-w64-mingw32-pkg-config'
+
+[host_machine]
+system = 'windows'
+cpu_family = 'x86'
+cpu = 'i686'
+endian = 'little'
+
+[properties]
+prebuilt_ffmpeg_shared = 'ffmpeg-4.0-win32-shared'
+prebuilt_ffmpeg_dev = 'ffmpeg-4.0-win32-dev'
+prebuilt_sdl2 = 'SDL2-2.0.8/i686-w64-mingw32'