Export optional component availability to GMake

Provide new GMake function g4gmake_option_variable to set Geant4
variables indicating the availability or otherwise of an optional
component (e.g. Qt support). Implement a GNU make construct to convert a
CMake boolean false value to a GNU make empty value, with CMake boolean
true values converted to a GNU make value of 1. This allows easy export
of CMake booleans to GMake so that @VAR@ expansion variables may use
the CMake values directly.

Implement a trivial template Makefile to test the Geant4GMake system.
TODO: Add the running of this as a CTest.