* m4/pthread-mutex.m4 (gl_PTHREAD_MUTEX): Test whether
pthread_mutexattr_getrobust exists. If not, define
PTHREAD_MUTEXATTR_ROBUST_UNIMPLEMENTED.
* lib/pthread-mutex.c (pthread_mutexattr_getrobust,
pthread_mutexattr_setrobust): Define also if <pthread.h> exists but
PTHREAD_MUTEXATTR_ROBUST_UNIMPLEMENTED.
* modules/pthread-mutex (configure.ac): Compile pthread-mutex.c also
when <pthread.h> exists but pthread_mutexattr_getrobust needs a gnulib
definition.