build: Fix SystemTap build to disable semaphores as before

At some point, upstream SystemTap changed from using a
STAP_HAS_SEMAPHORES preprocessor variable for this, to using
_SDT_HAS_SEMAPHORES instead. We need to update our build system to
disable that as well.

The original discussion about use of semaphores is here:

https://bugzilla.gnome.org/show_bug.cgi?id=606044

This was breaking the build with -flto enabled, either because -flto
doesn’t work with semaphores.

https://bugzilla.gnome.org/show_bug.cgi?id=768198