opam, aspcud: init packages for external solver (#16938)

The opam package manager relies on external solvers to determine package
management decisions it makes related to upgrades, new installations,
etc.

While, strictly speaking, an external solver is optional, aspcud is
highly recommended in documentation. Furthermore, even having a
relatively small number of packages installed quickly causes the limits
of the interal solver to be reached (before it times out).

Aspcud itself depends on two programs from the same suite: gringo, and
clasp.

On Darwin, Boost 1.55 (and thus Gringo) do not build, so we only support
Aspcud on non-Darwin platforms.