build: drop the getopt-posix gnulib module

build: drop the getopt-posix gnulib module

The getopt-posix module fixes a problem with optind being incorrectly
set after a failed option parse. It was also previously used to allow
the bhyve driver to access a private internal reentrant getopt impl.
None of this matters to libvirt code any more.

This partially reverts

  commit b436a8ae5ccb04f8cf893d882d52ab5efc713307
  Author: Fabian Freyer <fabian.freyer@physik.tu-berlin.de>
  Date:   Thu Jun 9 00:50:35 2016 +0000

    gnulib: add getopt module

Reviewed-by: Ján Tomko <jtomko@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>