gmain: abort if monotonic time is unsupported

We now depend on CLOCK_MONOTONIC, but it's possible that people may
attempt to run GLib on systems where it isn't supported at runtime.

Check the return value of clock_gettime() and abort() if it fails in
order to save these people from wasting time on debugging a tricky
issue.

https://bugzilla.gnome.org/show_bug.cgi?id=670144
GCond: check result of pthread_condattr_setclock()

Make sure this call succeeds, aborting if it doesn't

This will prevent people from having to waste time chasing down the problems
that would otherwise be caused by this silent failure.
g_main_context_wait: add a critical to detect use

Due to its unusual interface, I suspect that nobody is using
g_main_context_wait() but there is no way to know.

Add a critical notice that will be displayed if anyone calls the
function, asking them to file a bug with us.

We'll let this go out with the 2.40 release and see if we get a response
before we proceed with actually breaking the functionality.