Vendor dependencies

Let's see how I like this workflow.

vendor/cxx/tools/buck/prelude/toolchains/ocaml.bzl

@@ -0,0 +1,57 @@
+# Copyright (c) Meta Platforms, Inc. and affiliates.
+#
+# This source code is licensed under both the MIT license found in the
+# LICENSE-MIT file in the root directory of this source tree and the Apache
+# License, Version 2.0 found in the LICENSE-APACHE file in the root directory
+# of this source tree.
+
+load(
+    "@prelude//ocaml/providers.bzl",
+    "OCamlPlatformInfo",
+    "OCamlToolchainInfo",
+)
+
+def _system_ocaml_toolchain_impl(_ctx):
+    """
+    A very simple toolchain that is hardcoded to the current environment.
+    """
+
+    return [
+        DefaultInfo(
+        ),
+        OCamlToolchainInfo(
+            ocaml_compiler = RunInfo(args = ["ocamlopt.opt"]),
+
+            # "Partial linking" (via `ocamlopt.opt -output-obj`) emits calls to
+            # `ld -r -o`. If not `None`, this is the `ld` that will be invoked;
+            # the default is to use whatever `ld` is in the environment. See
+            # [Note: What is `binutils_ld`?] in `providers.bzl`.
+            binutils_ld = None,
+
+            # `ocamlopt.opt` makes calls to `as`. If this config paramter is
+            # `None` those calls will resolve to whatever `as` is in the
+            # environment. If not `None` then the provided value will be what's
+            # invoked.
+            binutils_as = None,
+            dep_tool = RunInfo(args = ["ocamldep.opt"]),
+            yacc_compiler = RunInfo(args = ["ocamlyacc"]),
+            interop_includes = None,
+            menhir_compiler = RunInfo(args = ["menhir"]),
+            lex_compiler = RunInfo(args = ["ocamllex.opt"]),
+            libc = None,
+            ocaml_bytecode_compiler = RunInfo(args = ["ocamlc.opt"]),
+            # `ocamldebug` is bytecode intended to be run by `ocamlrun`. There
+            # is no "debugger" executable (but then `debug` is not referenced by
+            # the ocaml build rules) so `None` will do for this.
+            debug = None,
+            warnings_flags = "-4-29-35-41-42-44-45-48-50-58-70",
+            ocaml_compiler_flags = [],  # e.g. "-opaque"
+        ),
+        OCamlPlatformInfo(name = "x86_64"),
+    ]
+
+system_ocaml_toolchain = rule(
+    impl = _system_ocaml_toolchain_impl,
+    attrs = {},
+    is_toolchain_rule = True,
+)