core: support host gcc of the future

When we do a release, we know only of a set of gcc versions that the
host may have. But in the future, distributions with newer gcc versions
may show up.

Currently, we do not recognise those versions, and thus we do as if they
were older than the oldest we know of. This means that a set of packages
become unselectable, when they should be.

We fix that by capping the detected version to the highest we know of.

Reported-by: gargar_ on IRC
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Peter Korsgaard <peter@korsgaard.com>
Cc: Arnout Vandecappelle <arnout@mind.be>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>