macOS is shipped with an old non-enhanced version of getopt and it
doesn't support options used by run_tests.sh. Proper version of getopt
is available from homebrew but it has to be added to PATH before invoking
run_tests.sh.  Halt ./configure if enhanced getopt can't be found.
Based on a patch by Roman Bolshakov <r.bolshakov@yadro.com>.

Cc: Cameron Esfahani <dirty@apple.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>