Bump src_ext Dune to 1.6.3

[dra27]

Allows compilation with OCaml 4.08.0 - this is for 2.0 branch only, master should be bumped to latest Dune (though there's some work to deal with the jbuild deprecation warning needed).

[avsm]

Thanks, I think it would be useful to cut this as a 2.0.5 point release

[avsm]

Incidentally, why not just go up to latest Dune 1.10.x?

[dra27]

Because the jbuild deprecation warning breaks building the man pages, and I'm still working on that fix!

[dra27]

@avsm - presumably also OK to cut it as 2.0.4-1 release tarball and submit to homebrew, etc. directly? It feels odd to cut a release with no actual features or bug fixes.

[avsm]

Just cherry pick in #3886 and you have a bugfix release :-) I think it's far easier for packaging if we're consistent with our versioning. For instance the -1 suffix is also used by debian as an extra version so it starts to clash.

[dra27]

The version wouldn't change it - it's just a new release tarball and any version bumping in Debian is theirs to do. But you're right that a quick release can also be cut.

[avsm]

Generally speaking, I view a build system upgrade as a reasonably major change -- there may be unexpected regressions or side effects that we don't anticipate. From a packagers perspective, newer versions of Dune may do something surprising that affects their packaging infrastructure and require some tweaks. A "recut" of a point release should be reserved for when we are absolutely sure there are no weird side effects (for example, documentation fixes or README changes or things of that nature).