Up the minimum system to build FreeBSD current to 8.0-RELEASE. The
issues with vendors that needed 7.x support have been resolved. Many
vendors are still using 8.x build platforms, however, so bumping this
up to 9.0 will have to wait until that is resolved. Actual support for
building from 8.x still relies on those vendors fixing bugs that are
present as most developers have moved onto 9.x or newer platforms.

Reviewed by: marcel@