nwrap: Add compatibility for musl-libc 1.1

nwrap: Add compatibility for musl-libc 1.1

The major difference is that getpwent_r and getgrent_r do not exist in
musl-libc. Hence it is sufficient to compile the wrappers for those only
conditionally.

Tested with musl-libc 1.1.15-r5 on Alpine Linux 3.5. Also confirmed to
not break with GNU libc 2.24-8 on Debian "Stretch" / 9.

Signed-Off-By: Dennis Schridde <dennis.schridde@uni-heidelberg.de>
Reviewed-by: Andreas Schneider <asn@samba.org>
Reviewed-by: Michael Adam <obnox@samba.org>