ctdb: fix compilation on systems with glibc robust mutexes

ctdb: fix compilation on systems with glibc robust mutexes

On older systems like SLES 11 without POSIX robust mutexes, but with glib robust
mutexes where all the functions are available but have a "_np" suffix,
compilation fails in:

ctdb/tests/src/test_mutex_raw.c.239.o: In function `worker':
/root/samba-4.10.6/bin/default/../../ctdb/tests/src/test_mutex_raw.c:129: undefined reference to `pthread_mutex_consistent'
ctdb/tests/src/test_mutex_raw.c.239.o: In function `main':
/root/samba-4.10.6/bin/default/../../ctdb/tests/src/test_mutex_raw.c:285: undefined reference to `pthread_mutex_consistent'
/root/samba-4.10.6/bin/default/../../ctdb/tests/src/test_mutex_raw.c:332: undefined reference to `pthread_mutexattr_setrobust'
/root/samba-4.10.6/bin/default/../../ctdb/tests/src/test_mutex_raw.c:363: undefined reference to `pthread_mutex_consistent'
collect2: ld returned 1 exit status

This could be fixed by using libreplace system/threads.h instead of pthreads.h
directly, but as there has been a desire to keep test_mutex_raw.c standalone and
compilable without other external depenencies then libc and libpthread, make the
tool developer build only. This should get the average user over the cliff.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=14038
RN: Fix compiling ctdb on older systems lacking POSIX robust mutexes

Signed-off-by: Ralph Boehme <slow@samba.org>
Reviewed-by: Martin Schwenke <martin@meltin.net>