mandoc: fix bootstrapping on Linux with Clang 16

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