variables for selecting builds based on compiler/arch/os

[avsm]

It would be useful to be able to select build rules based on some variables other than the presence of libraries. For example:

• ocaml-version: Merlin needs to select a module based on the current switch
• arch: C bindings need to select arm/x86 specific modules (nocrypto)
• os: platform bindings (e.g. mirage-xen/mirage-unix)

For each of these, it is possible to do a configurator/jbuild-generation pass, but the big advantage of having variable rules directly is that they work when embedded in a Duniverse of related packages. For example, with a manually assembled Mirage duniverse, the default dune build rule doesnt work as it selects the Linux- and macOS-only packages simultaneously. If Dune could handle these parameters as part of the build scanning process, then package embedding works great.

As a longer term consideration, this also expands our ability to generate accurate package management scripts as well -- for example, opam files could have accurate available: constraints added based on these build rules as well.

[dra27]

Related to #397

[avsm]

The code from opam-depext could be vendored in to help with the os variables -- there's a lot of fairly clean code there that generates the variables in opam2.