Don't use sysconf(_SC_NGROUPS_MAX) MacOS

Fri Oct 5 11:00:21 UTC 2018

New comment by bjacke on Samba Github repository

https://github.com/samba-team/samba/pull/210#issuecomment-427325885
Comment:
setting this fixed to 128 is definetely not right, a configure test is needed and the actual value should come from somewhere and not out of nothing. Please open a bugzilla bug for that and propose a patch there. Github is not used by Samba any more, even if the README file still contains that false informatin.