Medium-size FFMpeg binaries with OpenH264 support was built from current git sources 2023.01.29.
Building on Windows 7 was pretty messy, here is my attempt to list required steps.

Media-autobuild_suite was used to install msys64 (latest version of msys64 cannot be installed on Windows 7 directly, but this way still works) and build ffmpeg once, fetching dependencies.

OpenH264 was installed: pacman -S mingw-w64-i686-openh264

OpenH264 includes "wels" copied to /mingw32/include/vc.
File openh264-2.3.1-win32.dll renamed to openh264.dll.
File openh264.lib generated from openh264.dll using dll2lib.bat and copied to D:\MinGW.

yasm copied from MINGW/msys32 to msys64 (just to solve some problem with paths)

Started Command Prompt for VS2017
Run: set LIB=D:\MinGW;%LIB%
Run: D: and D:\MinGW\msys64\msys2_shell.cmd -full-path -mingw -where ..

In msys shell:
Run: cd /build/ffmpeg-git
Run: ./configure --toolchain=msvc --disable-everything --disable-doc --disable-programs --disable-encoders --disable-muxers --disable-debug --enable-shared --arch=x86 --enable-shared --enable-decoder=aac,aac_fixed,aac_latm,ac3,ac3_fixed,adpcm_g722,av1,eac3,h261,h263,h264,hevc,libopenh264,mp1,mp2,mp3,mp3adu,mjpeg,mpeg1video,mpeg2video,mpeg4,mpegvideo,opus,pcm_alaw,pcm_ulaw,pcm_s16le,vp8,vp9 --enable-encoder=h263 --enable-libopenh264 --enable-encoder=libopenh264 --enable-encoder=mpeg4 --enable-network --enable-protocol=file,tcp,udp,rtp,http --enable-demuxer=aac,ac3,asf,avi,g722,h261,h263,h264,hevc,matroska,mjpeg,mov,mp3,mpegts,mpegvideo,ogg,pcm_alaw,pcm_ulaw,pcm_s16le,rtsp,vp8,vp9 --shlibdir=/local32/bin-video --enable-w32threads --enable-version3 --extra-ldflags="-DEBUG" --extra-cflags="-Zi -I/mingw32/include/vc"
Run: make


$ ./configure --toolchain=msvc --disable-everything --disable-doc --disable-programs --disable-encoders --disable-muxers --disable-debug --enable-shared --arch=x86 --enable-shared --enable-decoder=aac,aac_fixed,aac_latm,ac3,ac3_fixed,adpcm_g722,av1,eac3,h261,h263,h264,hevc,libopenh264,mp1,mp2,mp3,mp3adu,mjpeg,mpeg1video,mpeg2video,mpeg4,mpegvideo,opus,pcm_alaw,pcm_ulaw,pcm_s16le,vp8,vp9 --enable-encoder=h263 --enable-libopenh264 --enable-encoder=libopenh264 --enable-encoder=mpeg4 --enable-network --enable-protocol=file,tcp,udp,rtp,http --enable-demuxer=aac,ac3,asf,avi,g722,h261,h263,h264,hevc,matroska,mjpeg,mov,mp3,mpegts,mpegvideo,ogg,pcm_alaw,pcm_ulaw,pcm_s16le,rtsp,vp8,vp9 --shlibdir=/local32/bin-video --enable-w32threads --enable-version3 --extra-ldflags="-DEBUG" --extra-cflags="-Zi -I/mingw32/include/vc"
install prefix            /usr/local
source path               .
C compiler                cl.exe
C library                 msvcrt
ARCH                      x86 (generic)
big-endian                no
runtime cpu detection     yes
standalone assembly       yes
x86 assembler             yasm
MMX enabled               yes
MMXEXT enabled            yes
3DNow! enabled            yes
3DNow! extended enabled   yes
SSE enabled               yes
SSSE3 enabled             yes
AESNI enabled             yes
AVX enabled               yes
AVX2 enabled              yes
AVX-512 enabled           yes
AVX-512ICL enabled        yes
XOP enabled               yes
FMA3 enabled              yes
FMA4 enabled              yes
i686 features enabled     yes
CMOV is fast              no
EBX available             no
EBP available             no
debug symbols             no
strip symbols             no
optimize for size         no
optimizations             yes
static                    no
shared                    yes
postprocessing support    no
network support           yes
threading support         w32threads
safe bitstream reader     yes
texi2html enabled         no
perl enabled              yes
pod2man enabled           yes
makeinfo enabled          yes
makeinfo supports HTML    yes
xmllint enabled           yes

External libraries:
libopenh264             mediafoundation         schannel

External libraries providing hardware acceleration:
d3d11va                 dxva2

Libraries:
avcodec                 avfilter                avutil                  swscale
avdevice                avformat                swresample

Programs:

Enabled decoders:
aac                     av1                     libopenh264             mpeg1video              pcm_s16le
aac_fixed               eac3                    mjpeg                   mpeg2video              vp8
aac_latm                h261                    mp1                     mpeg4                   vp9
ac3                     h263                    mp2                     mpegvideo
ac3_fixed               h264                    mp3                     opus
adpcm_g722              hevc                    mp3adu                  pcm_alaw

Enabled encoders:
h263                    libopenh264             mpeg4

Enabled hwaccels:

Enabled parsers:
aac_latm                h263                    mpegaudio
ac3                     mpeg4video              vp9

Enabled demuxers:
aac                     g722                    hevc                    mp3                     pcm_alaw
ac3                     h261                    matroska                mpegts                  pcm_s16le
asf                     h263                    mjpeg                   mpegvideo               rm
avi                     h264                    mov                     ogg                     rtsp

Enabled muxers:

Enabled protocols:
file                    http                    rtp                     tcp                     udp

Enabled filters:

Enabled bsfs:
av1_frame_split         h264_mp4toannexb        vp9_superframe_split

Enabled indevs:

Enabled outdevs:

License: LGPL version 3 or later
