mandoc: Fix bootstrapping on macOS with Clang 16 / Apple Clang 15

macOS, like Linux, does not include an outer const qualifier for its
fts_open callback arguments, so -Wincompatible-function-pointer-types
also picks this up and breaks the build now Clang 16 makes it an error
by default. Extend the existing Linux support to fix this.

MFC after: 1 week

(cherry picked from commit ce312ef72d91888d332f7f042298001794b66914)