Disables Xkb at run-time

Please see Issue #184.  This code automatically disables Xkb at either run-time or compile-time when it is not available.  There is no longer a user-visible CMake option to disable Xkb.

In my previous pull request, I tried to use XKeycodeToKeysym, but that was deprecated, resulting in compiler warnings which precluded run-time disabling of Xkb.  Now GLFW can use XGetKeyboardMapping as a fallback, which is *not* deprecated.