This PR changes the semantic of == for spec so that:
hdf5++mpi == hdf5+mpi
won't hold true anymore. It also changes the constrain semantic, so that a
non-propagating variant always override a propagating variant. This means:
(hdf5++mpi).constrain(hdf5+mpi) -> hdf5+mpi
Before we had a very weird semantic, that was supposed to be tested by unit-tests:
(libelf++debug).constrain(libelf+debug+foo) -> libelf++debug++foo
This semantic has been dropped, as it was never really tested due to the == bug.
395c911689