[SV 46261] Use pselect() for jobserver where supported.

* Makefile.am, configure.ac: Check for pselect() and sys/select.h.
* main.c (main): Block SIGCHLD if we have pselect() support.
* posixos.c (jobserver_acquire): If we support pselect() then use
it to query the jobserver pipe, while also listening for SIGCHLD.
Also pselect() supports a timeout so avoid alarm() calls.
6 files changed