we pull this feat off by booting configure with a dummy spec. the proper
spec gets loaded subsequently.
note that it was necessary to move the cache loading after processing
the early checks (from which the spec handling is triggered). this is
just fine, as the cache is needed only by tests, which are forbidden at
this stage by definition.
Change-Id: I5120e25a8bf05fb8cc5485fd93cf6387301089aa
Reviewed-by: Jake Petroules <jake.petroules@qt.io>
ab0cc3055d