cross-tools: fts has different types for cmp routine

fts has different types for its compare rotuine. Other systems, not
4.4BSD based, have a non-const version. Before we tested against
__GLIBC__, but now we test against __linux__ because that's Linux's API
and musl doesn't define __GLIBC__.

In addition, link against libftl on this platform since musl doesn't
include ftl routines in libc, but rather in libftl.

Co-authored-by: Val Packett <val@packett.cool>
Sponsored by: Netflix
Pull Request: https://github.com/freebsd/freebsd-src/pull/1066
Reviewed by: val_packett.cool
Differential Revision: https://reviews.freebsd.org/D45349

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

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

mandoc: import version 1.14.6

MFC after: 3 weeks

Update mandoc to 1.14.5

MFC after: 2 weeks

Import mandoc 1.14.4

Approved by: re (kib)
MFC after: 1 week