This file will describe how we build the FFmpeg executable file. 
For our build only 32-bit version is used.

About install and prepare msys2
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1) download and install msys2 (32-bit Version) from https://www.msys2.org/
2) follow the update/install procedure described on that page - that is call pacman some times with according parameters;
   NOTE, at some point in this process I had to correct Start menu msys2 shortcut to .bat file, because the file became .cmd, sth like [path]\msys64\msys2_shell.cmd
3) as https://ffmpeg.org/platform.html#Windows mentions, call
3a)# normal msys2 packages
   'pacman -S make pkg-config diffutils'
3b)# mingw-w64 packages and toolchains
   'pacman -S mingw-w64-i686-nasm mingw-w64-i686-gcc mingw-w64-i686-SDL2'

This was enough to complete mingw64/msys2 installation for me.


About building ffmpeg:
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1) enter the msys2 command line (invoking .cmd from shortcut). 
   For example via batch-file you have to write into it:
   'yourPathTo\msys2_shell.cmd -mingw32 -where yourPathToTheExtractedFolder\FFmpeg_source\'
2) in the msys2 command line call './configure' for configuring the build is done with it; its command line switches can be found in its text, or are listed with --help option
   Note: ./configure doesn't show anythig for a while, it's slow, just wait.
3) this is an optional step we did; the fact is that I wanted to build standalone ffmpeg.exe, without any mingw/msys dll dependencies; to achieve this, I hide (e.g. rename) according .dll.a files, so the linker links libraries statically; the libraries were libpthread.dll.a, libwinpthread.dll.a, libbz2.dll.a, libSDL.dll.a, libz.dll.a and libiconv.dll.a
4) in the msys2 command line call 'make' for building itself; it also has some options, if necessary.