Add bounds for removed dune ctypes 0.1 extension
The ctypes extension in Dune is documented as experimental (and has
version 0.x) so is not subject to the same compatibility as the rest of
Dune.
Dune 3.11.0 removed versions 0.1 and 0.2 of this extension, so packages
using it need to build with `dune < 3.11`.
Affected packages need to migrate to version 0.3. This should be just a
matter of changing the version of the extension in `dune-project`, but:
- it requires `(lang dune 3.7)`
- if C code refers to local include paths, these are now relative to the
where the stanza is located. This avoids the need to specify paths
twice. See ocaml/dune#6883.