How to compile Windows ffmpeg

1. Install MSYS2 from https://msys2.github.io/
2. Install Mingw-W64 `pacman -S mingw-w64-i686-gcc`
3. Install git autoconf automake libtool make pkg-config
   `pacman -S git`
   `pacman -S autoconf`
   `pacman -S automake`
   `pacman -S libtool`
   `pacman -S make`
   `pacman -S pkg-config`
4. yasm：http://yasm.tortall.net/，change to yasm.exe，move to C:\msys32\usr\bin
5. git clone ffmpeg openssl fdk-aac
6. ...