> AFAICT there is something missing in the patch: I do not see any compilation of math/logbq.c and indeed no trace of logbq in libquadmath. What I am missing?

Maybe you didn’t regenerate the Makefile.in?
The patch was sent without this regenerated file, as is (as I understand) the custom on gcc-patches.

Attached is the full diff, including Makefile.in.

FX