[upstream] update to tag 2.40.0
[packaging] update packaging to 2.40.0
[clock] use G_LOCK env var to allow requesting CLOCK_BOOTTIME for monotonic time

CLOCK_MONOTONIC has the drawback that it does not increment during
suspend. Some applications may need timeouts that follow wall clock
time even if the system is suspended; provide a way to do this by
adding an environment variable G_CLOCK that determines which clock
is used for monotonic time.