Improve documentation about build time module selection

[AllanBlanchard]

Basic problem

Depending on some compile time information (OS, library availability, ...), I want to select the right module for implementing a particular API, what is the idiomatic way to proceed?

Existing elements of documentation

• Alternatives via select.

  https://dune.readthedocs.io/en/stable/reference/library-dependencies.html#alternative-dependencies

  However it only works for library availability.

• Virtual libraries

  https://dune.readthedocs.io/en/stable/variants.html

  However, it is unclear to me whether we can really select a variant at compile time depending on a build time information.

[AllanBlanchard]

Poke @emillon