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