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.

prebuilt-deps/Makefile

@@ -0,0 +1,40 @@
+.PHONY: prepare-win32 prepare-win64 \
+	prepare-ffmpeg-shared-win32 \
+	prepare-ffmpeg-dev-win32 \
+	prepare-ffmpeg-shared-win64 \
+	prepare-ffmpeg-dev-win64 \
+	prepare-sdl2 \
+	prepare-adb
+
+prepare-win32: prepare-sdl2 prepare-ffmpeg-shared-win32 prepare-ffmpeg-dev-win32 prepare-adb
+prepare-win64: prepare-sdl2 prepare-ffmpeg-shared-win64 prepare-ffmpeg-dev-win64 prepare-adb
+
+prepare-ffmpeg-shared-win32:
+	@./prepare-dep https://ffmpeg.zeranoe.com/builds/win32/shared/ffmpeg-4.0-win32-shared.zip \
+		530c92df0ca14c35901b4b681847d62da3c50a0cc9b7ced37b04968f6b5c243d \
+		ffmpeg-4.0-win32-shared
+
+prepare-ffmpeg-dev-win32:
+	@./prepare-dep https://ffmpeg.zeranoe.com/builds/win32/dev/ffmpeg-4.0-win32-dev.zip \
+		e2f5200b5e73c4d0abb9b89c4ffc0438f92a0aadc54c81cf57e18c81a9f11c6b \
+		ffmpeg-4.0-win32-dev
+
+prepare-ffmpeg-shared-win64:
+	@./prepare-dep https://ffmpeg.zeranoe.com/builds/win64/shared/ffmpeg-4.0-win64-shared.zip \
+		8fe2d344463dbefc2db4239a4203a55ed0324faceaae57276a40c4fabda84c37 \
+		ffmpeg-4.0-win64-shared
+
+prepare-ffmpeg-dev-win64:
+	@./prepare-dep https://ffmpeg.zeranoe.com/builds/win64/dev/ffmpeg-4.0-win64-dev.zip \
+		facced738eabfc53fa92834dea8b24426f64db61298688fed480145945be07fa \
+		ffmpeg-4.0-win64-dev
+
+prepare-sdl2:
+	@./prepare-dep https://libsdl.org/release/SDL2-devel-2.0.8-mingw.tar.gz \
+		ffff7305d634aff5e1df5b7bb935435c3a02c8b03ad94a1a2be9169a558a7961 \
+		SDL2-2.0.8
+
+prepare-adb:
+	@./prepare-dep https://dl.google.com/android/repository/platform-tools_r27.0.1-windows.zip \
+	    880662adfb0d6911ff250b9e13930ae1a4110fc36d5866afd4f8f56d935f7939 \
+		platform-tools