Qt5: use pthread synchronization functions

The Genode-specific implementation of 'QWaitCondition' contains a race
condition which can make the 'qt5_samegame' application hang on Fiasco.OC.

Since most of the pthread synchronization functions needed by the
UNIX-specific implementation of 'QWaitCondition' and 'QMutex' are
available now, we can use these now instead of fixing and keeping
the Genode-specific implementation.

Fixes #993.