From opencv/3rdparty/ffmpeg/readme.txt:
On Windows OpenCV uses pre-built ffmpeg binaries, built with proper flags (without GPL components) and wrapped with simple, stable OpenCV-compatible API. The binaries are opencv_ffmpeg.dll (version for 32-bit Windows) and opencv_ffmpeg_64.dll (version for 64-bit Windows).
See build_win32.txt for the build instructions, if you want to rebuild opencv_ffmpeg*.dll from scratch.
There is no build_win32.txt. The old version removed from repository (why?) gives some hints, but the current process clearly supports only the usage of the downloaded precompiled variants. At least as far as I can tell from the cmake infrastructure, even though videoio/src/cap_ffmpeg* still contains wrapper sources. Could someone clarify the necessary steps for MSVC-based Windows builds, in order to utilize custom-build versions of the ffmpeg libraries?
I'm building ffmpeg from sources here. Nowadays, this is much easier, even with a MSVC toolchain. In my opinion everyone should be encouraged to build a system with the components, fitting his needs best. Licensing issues are certainly not neglectable at times, but it should in my opinion not lead to obfuscated or quirky technical solutions.