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/.gitignore

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+*
+!/.gitignore
+!/Makefile
+!/prepare-dep

prebuilt-deps/Makefile

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+.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

prebuilt-deps/prepare-dep

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+#!/bin/bash
+set -e
+url="$1"
+sum="$2"
+dir="$3"
+
+checksum() {
+    local file="$1"
+    local sum="$2"
+    echo "$file: verifying checksum..."
+    echo "$sum  $file" | sha256sum -c
+}
+
+get_file() {
+    local url="$1"
+    local file="$2"
+    local sum="$3"
+    if [[ -f "$file" ]]
+    then
+        echo "$file: found"
+    else
+        echo "$file: not found, downloading..."
+        wget "$url" -O "$file"
+    fi
+    checksum "$file" "$sum"
+}
+
+extract() {
+    local file="$1"
+    echo "Extracting $file..."
+    if [[ "$file" == *.zip ]]
+    then
+        unzip -q "$file"
+    elif [[ "$file" == *.tar.gz ]]
+    then
+        tar xf "$file"
+    else
+        echo "Unsupported file: $file"
+        return 1
+    fi
+}
+
+get_dep() {
+    local url="$1"
+    local sum="$2"
+    local dir="$3"
+    local file="${url##*/}"
+    if [[ -d "$dir" ]]
+    then
+        echo "$dir: found"
+    else
+        echo "$dir: not found"
+        get_file "$url" "$file" "$sum"
+        extract "$file"
+    fi
+}
+
+get_dep "$1" "$2" "$3"