Add BST_REQUIRES_PREVIOUS_SOURCE_STAGE option

This is an element option that allows sources to be staged more
seperately where possible rather than on a per element option. When
there is a source that has this option set to true, all previously
listed sources will be cached with this, and sources listed afterwards
will be cached individually. If no sources have this option, all sources
will be cached seperately.

Fixes #892
4 files changed