Bug 1516325 - Use poll() in NSPR on MacOS. r=glandium

This is being changed in Firefox's build config rather than NSPR's
to avoid possibly introducing regressions into other NSPR users due
to bugs in MacOS's poll, such as the POLLPRI issue addressed in the
previous patch.  (There is also a known bug when calling poll() with
zero descriptors, but PR_Poll already has code to call PR_Sleep instead
in that case.)

Depends on D21321

Differential Revision: https://phabricator.services.mozilla.com/D21322

--HG--
extra : moz-landing-system : lando

config/external/nspr/pr/moz.build

@@ -53,6 +53,7 @@ elif CONFIG['OS_TARGET'] == 'Darwin':
         HAVE_BSD_FLOCK=True,
         HAVE_SOCKLEN_T=True,
         HAVE_POINTER_LOCALTIME_R=True,
+        _PR_POLL_AVAILABLE=True,
     )
     SOURCES += [
         '/nsprpub/pr/src/md/unix/darwin.c',