[Semaphore] make default constructor more explicit

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
semaphore count to 0.

The default constructor using clang and libc++ does not initialize the
semaphore count member and thus causing the CancelFutureTest to misbehave.

emper/Semaphore.hpp

@@ -17,7 +17,7 @@ class Semaphore {
 	std::mutex mutex;
 
  public:
-	Semaphore() = default;
+	Semaphore() : Semaphore(0){};
 	Semaphore(unsigned int count) : count(count){};
 
 	auto acquire() -> bool {