getopt-posix: Fix build failure when using ac_cv_header_getopt_h=no. Reported by Christian Ehrhardt <christian.ehrhardt@canonical.com> and Daniel P. Berrange <berrange@redhat.com>. * lib/unistd.in.h (getopt): Don't attempt to avoid namespace pollution on glibc systems. The getopt-pfx-core.h file declares exactly what unistd.h needs, nothing more.