Export version number to gmk configuration

Add Geant4_VERSION variable to main CMake script to hold "Geant4"
version. Export this to the GMake configuration file. Add a target
to the testing Makefile to print this number out.
Correct function name in g4gmake_config_variable
Rename g4gmake_option_variable impl function
Refactor gmake reporting into Geant4GMakeTargets

Move reporting of Geant4 GMake configuration/option variables from the
testing Makefile into Geant4GMakeTargets.gmk.in. Split reporting into
a target for reporting configuration and one for reporting optional
component availability. Provide overall "geant4_help" target that simply
depends on these two targets, so providing an overall report.
Implement GMake function(s) for defining datasets

Provide Geant4GMake.gmk.in with a callable function to define a Geant4
dataset. When called with the dataset (environment variable) name and
default path, check for definition of name. If the name is defined,
simply append it to the list of known names. This ensures the user
can still define the datasets via the environment, command line or
other makefile. If the name is not defined, append it the the list of
names, and set its value to the supplied default path.

For now, just define the G4LEDATA variable with a dummy value for
checking.

Add a geant4_datasets target in the targets file to report the dataset
names, where their value comes from, and the value itself.