Remove command_queue::enqueue_wait_for_event() method

This remove the enqueue_wait_for_event() method from the
command_queue class as the clEnqueueWaitForEvents() function
has been deprecated in OpenCL 1.2.
Fix command_queue::enqueue_barrier() for OpenCL 1.2

This changes the enqueue_barrier() method in the command_queue
class to use clEnqueueBarrierWithWaitList() instead of the
deprecated clEnqueueBarrier() function when compiling with
OpenCL 1.2.
Fix command_queue::enqueue_marker() for OpenCL 1.2

This changes the enqueue_marker() method in the command_queue
class to use clEnqueueMarkerWithWaitList() instead of the
deprecated clEnqueueMarker() function when compiling with
OpenCL 1.2.