Fix Event implementation

On non-windows platforms this just used a semaphore,
which meant that multiple calls to signal() would
result in wait() returning multiple times.

src/threading/CMakeLists.txt

@@ -1,4 +1,5 @@
 set(JTHREAD_SRCS
+	${CMAKE_CURRENT_SOURCE_DIR}/event.cpp
 	${CMAKE_CURRENT_SOURCE_DIR}/mutex.cpp
 	${CMAKE_CURRENT_SOURCE_DIR}/thread.cpp
 	${CMAKE_CURRENT_SOURCE_DIR}/semaphore.cpp