math/why3: Release port

I only care about math/why3-gpl, which has been decoupled from why3 and
has already diverged. Before resetting MAINTAINER, I reintegrated the
Makefile.common file (only used by this port) into the main Makefile. In
the process, some options placeholders were lost but in all probability
these options can't be built without serious work on external ports.