Tidy up dune files

[emillon]

In the spirit of #246:

• Fix warnings 27 and 32

There was a link to an old dune issue but it seems that these were hiding some problems.

• Use standard flags

To the best of my knowledge, it is best practice to let dune deal with flags.

• Use dune shortcuts

Small improvements by using shortcuts allowed by dune.

[tmcgilchrist]

Looks good to me, just failing on the 4.12 builds as expected.

[avsm]

Thanks!