Screen/Linux: Add minimal code to allow bringup of non-X11 display backends.

This simply makes sure Screen() doesn't shutdown completely if it
can't detect/connect to any X11 displays. It also turns all ScreenGlue
functions into no-ops which are currently not supported without a
X11 connection.

Screen for 64-Bit Octave on Linux rebuilt. So far, it comes up on
GBM backend from a console without crashing (immediately). However,
no onscreen window is displayed, apparently GBM calls go to Nirvana.

Babysteps...