build: Define _NPTH_NO_RWLOCK when we can't find pthread_rwlock_t.
* configure.ac (INSERT_NO_RWLOCK): New. * src/npth.h.in (@INSERT_NO_RWLOCK@): Use.
This change allows nPth to be compiled on old system with no
pthread_rwlock_t support.
- GnuPG-bug-id: T4306
- Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>