[glib] use CLOCK_BOOTTIME for monotonic time if requested.

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.
Hmm. Will there be any side effects to adding an exported symbol to glib? Could that break some kind of compatibility we care about?

I can't think of anything..