Unfortunately, for me it still doesn't work :(.
My full build log is at https://gist.github.com/eht16/9423377071e053555d27ab01838b2846, I just stripped the compiler warnings on C code which is most probably not relevant for the Meson changes.
First, there is a warning when configuring in https://gist.github.com/eht16/9423377071e053555d27ab01838b2846#file-gistfile.... Might be harmless, I cannot judge.
Beginning with https://gist.github.com/eht16/9423377071e053555d27ab01838b2846#file-gistfile... the real errors start. It seems `doxygen` is looking in the build folder for the source files.
In https://gist.github.com/eht16/9423377071e053555d27ab01838b2846#file-gistfile..., the `scripts/gen-api-gtkdoc.py` script is looked up in a hardcoded path `../` which obviously only works if one builds in a sub directory of the source tree but cannot work in full out of source tree builds.
I don't think we should merge this PR at this stage.