[build] Preliminary support for building Coq with `dune`.

[Dune](https://github.com/ocaml/dune) is a compositional declarative
build system for OCaml. It provides automatic generation of
`version.ml`, `.merlin`, `META`, `opam`, API documentation; install
management; easy integration with external libraries, test runners,
and modular builds.

In particular, Dune uniformly handles components regardless whether
they live in, or out-of-tree. This greatly simplifies cases where a
plugin [or CoqIde] is checked out in the current working copy but then
distributed separately [and vice-versa]. Dune can thus be used as a
more flexible `coq_makefile` replacement.

For now we provide experimental support for a Dune build. In order to
build Coq + the standard library with Dune type:

```
$ make -f Makefile.dune world
```

This PR includes a preliminary, developer-only preview of Dune for
Coq. There is still ongoing work, see
https://github.com/coq/coq/issues/8052 for tracking status towards
full support.

## Technical description.

Dune works out of the box with Coq, once we have fixed some modularity
issues. The main remaining challenge was to support `.vo` files.

As Dune doesn't support custom build rules yet, to properly build
`.vo` files we provide a small helper script `tools/coq_dune.ml`. The
script will scan the Coq library directories and generate the
corresponding rules for `.v -> .vo` and `.ml4 -> .ml` builds. The
script uses `coqdep` as to correctly output the dependencies of
`.v` files. `coq_dune` is akin to `coq_makefile` and should be able to
be used to build Coq projects in the future.

Due to this pitfall, the build process has to proceed in three stages:
1) build `coqdep` and `coq_dune`; 2) generate `dune` files for
`theories` and `plugins`; 3) perform a regular build with all
targets are in scope.

## FAQ

### Why Dune?

Coq has a moderately complex build system and it is not a secret that
many developer-hours have been spent fighting with `make`.

In particular, the current `make`-based system does offer poor support
to verify that the current build rules and variables are coherent, and
requires significant manual, error-prone. Many variables must be
passed by hand, duplicated, etc... Additionally, our make system
offers poor integration with now standard OCaml ecosystem tools such
as `opam`, `ocamlfind` or `odoc`. Another critical point is build
compositionality. Coq is rich in 3rd party contributions, and a big
shortcoming of the current make system is that it cannot be used to
build these projects; requiring us to maintain a custom tool,
`coq_makefile`, with the corresponding cost.

In the past, there has been some efforts to migrate Coq to more
specialized build systems, however these stalled due to a variety of
reasons. Dune, is a declarative, OCaml-specific build tool that is on
the path to become the standard build system for the OCaml ecosystem.

Dune seems to be a good fit for Coq well: it is well-supported, fast,
compositional, and designed for large projects.

### Does Dune replace the make-based build system?

The current, make-based build system is unmodified by this PR and kept
as the default option. However, Dune has the potential

### Is this PR complete? What does it provide?

This PR is ready for developer preview and feedback. The build system
is functional, however, more work is necessary in order to make Dune
the default for Coq.

The main TODOs are tracked at https://github.com/coq/coq/issues/8052

This PR allows developers to use most of the features of Dune today:

- Modular organization of the codebase; each component is built only
  against declared dependencies so components are checked for
  containment more strictly.
- Hygienic builds; Dune places all artifacts under `_build`.
- Automatic generation of `.install` files, simplified OPAM workflow.
- `utop` support, `-opaque` in developer mode, etc...
- `ml4` files are handled using `coqp5`, a native-code customized
  camlp5 executable which brings much faster `ml4 -> ml` processing.

### What dependencies does Dune require?

Dune doesn't depend on any 3rd party package other than the OCaml compiler.

### Some Benchs:

```
$ /usr/bin/time make DUNEOPT="-j 1000" -f Makefile.dune states
59.50user 18.81system 0:29.83elapsed 262%CPU (0avgtext+0avgdata 302996maxresident)k
0inputs+646632outputs (0major+4893811minor)pagefaults 0swaps

$ /usr/bin/time sh -c "./configure -local -native-compiler no && make -j states"
88.21user 23.65system 0:32.96elapsed 339%CPU (0avgtext+0avgdata 304992maxresident)k
0inputs+1051680outputs (0major+5300680minor)pagefaults 0swaps
```

1 files changed, 7 insertions, 0 deletions

diff --git a/lib/dune b/lib/dune
new file mode 100644
index 0000000000..232c208aa6
--- /dev/null
+++ b/lib/dune
@@ -0,0 +1,7 @@
+(library
+ (name lib)
+ (synopsis "Coq's Utility Library [coq-specific]")
+ (public_name coq.lib)
+ (wrapped false)
+ (modules_without_implementation xml_datatype)
+ (libraries threads coq.clib coq.config))