Some tests were made over Debian, Alpine and Fedora and the issue causing
the FTBFS is present only on Debian. The Meson finds the libobs, but fails
to build using it. Maybe it can be something related to #998853.
The upstream doesn't have a solution because he uses Alpine and the build
is perfect there (this bug is unreproducible outside of Debian).
My solution (or temporary solution): commonly, plugins for OBS are build
with CMake, so I made a patch[1] to use it instead of Meson. This solution
works fine and it is based in CMakeLists.txt files provided by Exeldro in
several plugins, like obs-move-transition[2].
[1] https://salsa.debian.org/debian/obs-time-source/-/blob/debian/master/debian/patches/010_use-cmake.patch
[2] https://salsa.debian.org/debian/obs-move-transition/-/blob/debian/master/CMakeLists.txt
Eriberto