Leave previous successefully built version of a library in the dist if new code fails to build

[svetlyak40wt]

This way, software will not be removed from new versions of the dist in case if something fails. Actually, a library will never be removed from a dist automatically – it will be possible to remove it only by hand.

This feature will solve an issue of loosing some system because of temporary errors or more serious cases – when its sources are not available anymore.

Idea for this feature was originated from #257