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/prepare-dep

@@ -0,0 +1,58 @@
+#!/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"