replace: Only link against librt if really needed

replace: Only link against librt if really needed

fdatasync() and clock_gettime() are provided by glibc on Linux, so there
is no need to link against librt. Checks have been added so if there are
platforms which require it are still functional.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=14140

Signed-off-by: Andreas Schneider <asn@samba.org>
Signed-off-by: Isaac Boukris <iboukris@gmail.com>
Pair-Programmed-With: Isaac Boukris <iboukris@gmail.com>
Reviewed-by: Matthias Dieter Wallnöfer <mdw@samba.org>
Reviewed-by: Alexander Bokovoy <ab@samba.org>