Hang in srfi-18.test

[Neil Jerram]

Just a heads up...

I'm seeing a hang in srfi-18.test, when running make check in master,
in the "exception handler installation is thread-safe" test.  It's not
100% reproducible, so looks like there's a race involved.

I think the problem is that wait-condition-variable is not actually
atomic in the way that it is supposed to be.  It unlocks the mutex,
then starts waiting on the cond var.  So it is possible for another
thread to lock the same mutex, and signal the cond var, before the
wait-condition-variable thread starts waiting.

I don't currently have a solution, but I'm working on it...

Regards,
    Neil