Factor configure-time gcc version checks (patch 1/4 for PR 7305)

[Richard Sandiford]

Zack Weinberg <zack@codesourcery.com> writes:
> I think this is a fine idea (though I cannot approve it) and I would
> like to encourage you also to break the version number proper and the
> date stamp out of gcc/version.c.

Yeah, a separate version file is probably a good idea, but I'm afraid
it's not something I'm likely to tackle myself, sorry.

Richard