We need to test the case of eventfd in the libc but no kernel support.
In order to do that, we add a separate compile of the GWakeup testcase
that interposes an 'eventfd' symbol that always returns -1 with errno
set. That will trigger the fallback case.
This new API allows watching a few select Unix signals;
looking through the list on my system, I didn't see anything
else that I think it'd reasonable to watch.
We build on the previous patch to make the child watch helper thread
that existed on Unix handle these signals in the threaded case.
In the non-threaded case, they're just global variables.
https://bugzilla.gnome.org/show_bug.cgi?id=644941
