tail: fix detection of closed stdout on macOS

tail: fix detection of closed stdout on macOS

* bootstrap.conf: We only need poll on Linux and AIX
where poll is not replaced.  Also resinstate dependence
on select so we can use it unconditionally.
* src/tail.c (check_output_alive): Reinstate use of select()
by default as poll was seen to be ineffective for this
application on macOS.
Fixes https://bugs.gnu.org/50714