Disables Xkb at run-time

Please see Issue #184.  This code automatically disables Xkb at either run-time or compile-time. Previously 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.