Allow use on systems which return ENOSYS for sem_init.
* src/npth.c: Include some more headers. (sceptre_buffer): New. (sceptre): Change to a pointer and init to sceptre_buffer. (enter_npth, leave_npth): Adjust. (try_sem_open): New. (npth_init): Check for ENOSYS and use sem_open.
This is for example the case on OS X and some AIX versions Thanks to
Patrick Brunschwig for finding this problem and suggesting a solution.