Provide an escape hatch for ".opt" detection

[jorisgio]

It appears that run stanza will always try to use .opt version of an executable if available, and i don't see any way to disable this.
this can be a problem for using for instance campl4 as you cannot invoke camlp4o so you need to build a cmxs.

[ghost]

We should probably change that indeed. You should be able to make it work by statically linking a camlp4 executable though.

[jorisgio]

to be honest that's a minor/niche issue, i barely opened an issue because that was unexpected behavior and this kind of "magic" semantic can be surprising. But it might not be worth fixing. You can work around it in several ways indeed.

[nojb]

This has been fixed in #2543