Tweak some NSPR variables for Darwin.

config/external/nspr/pr/moz.build

@@ -38,6 +38,8 @@ elif CONFIG['OS_TARGET'] in ('FreeBSD', 'OpenBSD', 'NetBSD'):
     SOURCES += ['/nsprpub/pr/src/md/unix/%s.c' % CONFIG['OS_TARGET'].lower()]
 elif CONFIG['OS_TARGET'] == 'Darwin':
     OS_LIBS += ['-framework CoreServices']
+    DEFINES['FD_SETSIZE'] = 10240
+    DEFINES['_DARWIN_UNLIMITED_SELECT'] = True
     DEFINES.update(
         DARWIN=True,
         HAVE_BSD_FLOCK=True,