Conditionally disable macOS incompatible tests

Conditionally disable macOS incompatible tests

Symbols _getgrent_r and _getpwent_r in
source4/torture/local/nss_tests.c are undefined in macOS. It seems
that checking HAVE_GETGRENT_R and HAVE_GETPWENT_R and conditionally
disabling those tests as suggested by hirochachacha in the referenced
bug allows samba on both `master` and `samba-4.7.1` to build properly
on macOS/darwin.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=11984

Signed-off-by: Will Haley <willhy@gmail.com>
Reviewed-by: Andreas Schneider <asn@samba.org>
Reviewed-by: Ralph Boehme <slow@samba.org>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>