GDB version as convenience variable

[Eli Zaretskii]

> Date: Mon, 25 Mar 2019 14:51:36 -0400
> From: Simon Marchi <simark@simark.ca>
> Cc: gdb-patches@sourceware.org, brobecker@adacore.com
> 
> comparing the output with gdb --version, or some other automated
> mean, would be perfect.

Well, I never wrote any tests for GDB, and I'm not sure I have a
system on which to run such a test.  So I hope someone else will add
this test.