maint: be defensive in avoiding gnulib's poll module

maint: be defensive in avoiding gnulib's poll module

* src/tail.c (check_output_alive): Add a guard that would
trigger on most platforms, to detect if we're using the
gnulib poll module.  That's currently problematic in the
way it emulates poll() using select() and would cause
issues on macOS and AIX at least as poll() is replaced there.