Make sthread_inside_simgrid static into libsthread in the (vain) hope that it'll...

Make sthread_inside_simgrid static into libsthread in the (vain) hope that it'll help the init/fini phases

but unfortunately the symptoms are exactly the same:
- default builds work well with sthread LD_PRELOAD'ed
- MC builds segfault once the second thread starts in this case
- ns-3 builds deadlock in dl_init with sthread

I'm still commiting this because I believe that breaking the hard
dependency of sthread -> simgrid goes in the right direction: If
sthread is alone in memory without simgrid (as it may happen at
init/fini phases), it's a rather bad idea for sthread to use a
variable that lives in simgrid symbols.

Maybe I should load simgrid dynamically with libdl instead of
depending on it to further untighten the dependency, but I'd prefer to
understand. Plus, it'd only help at init phase and ns-3 builds are not
the ones I'd prefer to fix here.