It could happen that externals were built incrementally, in case
something the ExternalProject depends on was updated.  To prevent this,
change the dependencies so that the UnpackedTarball is unpacked again
when the ExternalProject's dependencies are newer.  This is possible
without introducing a new target for the purpose due to the refactorings
in previous commits that enforce the name of UnpackedTarball.

Change-Id: Ie7a84064ec2ffc76175cd2b7792517e68664a461