Merged requested to merge aj46ezos/emper:fix-semaphore-for-libc++ into master
The CancelFutureTest fails for libc++ because it uses two
emper::Semaphores to coordinate the involved fibers.
Using gcc and stdlibc++ the default constructor initialized the
count to 0.
The default constructor using clang and libc++ does not initialize the
count member and thus causing the CancelFutureTest to misbehave.