* Lock X updates for all systems and lock around X cleanup at exit
All systems which could generate I/O interrupts on the X file descriptor need
to have locking enabled, or there is a high probability that the X library
will deadlock. This requirement may be removed in the future with restructuring
of the way that the interrupt driven I/O is handled.
Ensure that the appropriate XLOCK/XUNLOCK is done around the X window
closing code.
* Also lock X updates when building with CMake.
