Minor improvements for Build_info and install

[jberdine]

I noticed a few things about Build_info and related use of dune install that could be improved.

• [X] The https://dune.readthedocs.io/en/latest/dune-libs.html#build-info section led me to add dune.build-info to the libraries entry in my executable stanza, but from reading the dune tests it turned out that dune-build-info is needed instead.

• [X] The example code in that section doesn't compile since Build_info.V1.version now takes a unit arg.

• [ ] The Installing ... messages do not seem to respect e.g. --display=quiet.

• [ ] It would be nice to warn if setting --prefix=_build/install/... since doing so silently produces empty exes.

• [ ] dune install --help claims to support --watch but seems to ignore it.

[Kakadu]

Also build info introduces a compilation warning which is a little bit annoying

Warning 58 [no-cmx-file]: no cmx file was found in path for module Build_info__Build_info_data,
and its interface was not compiled with -opaque