I write a script to simplify the build. The script is in QtAV/scripts/build_ffmpeg.sh. It supports mingw gcc, msvc(2013), android, maemo5, maemo6(meego), sailfish os etc. Just put the script in ffmpeg source tree and run
./build_ffmpeg.sh
To cross build for android, maemo etc, you have to set some vars, for example NDK_ROOT for android. The vars can be set in config-android.sh, config-maemo5.sh etc.
make install prefix=some_dir
Open vs prompt, go to msys' bin dir and run
sh --login -i
Then your are ready to use bash environment and with VC environment. Then go to ffmpeg source dir and run './build_ffmpeg.sh vc'
if get error about gawk, just ignore make -ki
Your msys may lack of some command line tools that ffmpeg build system needs, such as pr, od, install, pkg-config, nasm. You can download one of build-ffmpeg-win-additinal-tools_msys2/gnuwin32.7z here: https://sourceforge.net/projects/qtav/files/depends/
extract and append it's bin dir to PATH. then you can build ffmpeg with gcc or vc.
If you build with vc and not vs2013, you have to download c99wrap
If we build FFmpeg using mingw gcc, the *,lib are also created and installed in /usr/local/bin. So VC linker can use those. *,lib are generated by vc toolchain's lib.exe and link.exe if they are found. Otherwise dlltool is used. You may need additional link flag /SAFESEH:NO when linking to libs created by dlltool to avoid link error.
If gcc does not create *.lib, for example, portaudio, then you can do it yourself.
dlltool -m i386 -d libportaudio-2.dll.def -l portaudio.lib -D libportaudio-2.dll
the def file can be found in lib/.lib for portaudio
cmake is recommended. It's not difficult.