A private copy of FFMPEG used in PPSSPP.
If on mac or linux, just run the corresponding build script (if it's written..).
If on Windows and building for Windows, use these instructions:
https://ffmpeg.org/platform.html#Microsoft-Visual-C_002b_002b
Then except instead of ./configure, just run ./windows_x86-build.sh.
If on Windows and building for Android, just run ./android-build.sh . Like for the Windows build, you may need real msys environment for that, and you may need to adjust some paths in android-build.sh.