Report XSpec version at command line

[AndrewSales]

It would be helpful if the version of XSpec could be report as part of the synopsis given at the command line, and probably also in the messages emitted at runtime.

(Many thanks as ever for your work on this tool, and apologies if I have missed a pre-existing issue or means to enable this.)

[AirQuick]

Agreed, it will be helpful.

I once implemented it and merged it into master (#1146), only to find later that I had to revert it due to its incompatibility with our release process.

Maybe we need some automation of updating the version number and/or releasing Oxygen add-on.