It appears that Clang 16 errors by default for the mismatched function
pointer types that are triggered by the fts callback (since glibc has a
callback type without the second const qualifier).
Fortunately, there is already code to handle glibc inside mandoc, we
just have to edit the checked-in config.h.
Reviewed By: imp, emaste
Differential Revision: https://reviews.freebsd.org/D42999
