GDB version as convenience variable

[Eli Zaretskii]

> Date: Mon, 25 Mar 2019 14:33:55 -0400
> From: Simon Marchi <simark@simark.ca>
> Cc: gdb-patches@sourceware.org, brobecker@adacore.com
> 
> > Hmm... how would you test that?
> 
> Something like
> 
> gdb_test "print \$_gdb_major" " = 8"
> gdb_test "print \$_gdb_minor" " = 4"

This would require changes with each new version.  I thought you had
in mind comparison with output of "gdb --version", suitably edited.