Add version command line option / #3735

[BaseMax]

Description

Add --version command line option which prints out the GAP version and exits. This command should be fast, i.e., be handled by the kernel before the library is loaded.

This would be helpful for scripts that just want to test if the installed GAP version is new enough.

Reffer to: #3735
Thanks to @fingolfin @hulpke

Further details

$ ./gap -v

KernelVersion 4.dev
BuildVersion 4.12dev-333-gfc65b03-dirty

Checklist for pull request reviewers

• proper formatting

If your code contains kernel C code, run clang-format on it; the
simplest way is to use git clang-format, e.g. like this (don't
forget to commit the resulting changes):

git clang-format $(git merge-base HEAD master)

• usage of relevant labels

  1. either release notes: not needed or release notes: to be added
  2. at least one of the labels bug or enhancement or new feature
  3. for changes meant to be backported to stable-4.X add the backport-to-4.X label
  4. consider adding any of the labels build system, documentation, kernel, library, tests

• runnable tests

• lines changed in commits are sufficiently covered by the tests

• adequate pull request title

• well formulated text for release notes

• relevant documentation updates

• sensible comments in the code

[ChrisJefferson]

This is better that hat we had before, but I think @fingolfin was thinking about putting this into the C level (there is a table around line 660 of system.c). Flags defined here have the advantage that they don't require GAP to start (which takes a while, and requires a working library)

[fingolfin]

Thank you for your contribution. However, this pull request in its current form does not address the issue adequately; as @ChrisJefferson explained, for that a change to the C kernel code will be necessary, in order to ensure GAP processes the --version argument before loading the library.