Enable support for Jane Street's internal Merlin configuration (#64)

* Enable support for Jane Street's internal Merlin configuration

Now Jane Street internal developers can get Merlin within the compiler
when building with `dune`.

Once we improve the internal tooling in a couple of ways (see
comments) we can simplify some of the way this is done.

* Use `ocamlc -where`

* Support `.local-merlin-binaries`

* Add `.local-merlin-*` files to `.gitignore`

Author: antalsz

.gitignore

@@ -291,3 +291,9 @@ _build
 
 /otherlibs/dynlink/natdynlinkops
 
+# Jane Street Merlin support
+/.for-jane-street-merlin/
+/.merlin-binaries
+/.ocaml-lib
+/.local-merlin-binaries
+/.local-ocaml-lib

.ocaml-bin

@@ -0,0 +1 @@
+Stub for Jane Street .merlin-binaries search

Makefile.common-jst

@@ -2,6 +2,9 @@
 # Build rules common to ocaml-jst and flambda-backend
 #
 
+# Get Merlin along with any Dune command
+dune := '$(CURDIR)/jane-street-merlin-setup.sh' ; '$(dune)'
+
 ws_boot   = --root=. --workspace=duneconf/boot.ws
 ws_runstd = --root=. --workspace=duneconf/runtime_stdlib.ws
 ws_main   = --root=. --workspace=duneconf/main.ws

jane-street-merlin-setup.sh

@@ -0,0 +1,27 @@
+#!/bin/bash
+
+cd "$(dirname "$0")"
+
+ocamlmerlin="$(which ocamlmerlin 2>&-)"
+ocamlmerlin_status=$?
+if [[ $ocamlmerlin_status -ne 0 ]]; then
+  echo >&2 'No ocamlmerlin on $PATH'
+  exit $ocamlmerlin_status
+fi
+
+set -e
+
+merlin_dir="$(dirname "$ocamlmerlin")"
+echo "$merlin_dir" > .local-merlin-binaries
+ocamlc -where > .local-ocaml-lib
+
+# When `.local-merlin-binaries` is fully supported (rather than just supported
+# in dev Emacs), we can drop the extra directory `.for-jane-street-merlin` as
+# well as the stub `jenga.conf` file.
+
+mkdir -p .for-jane-street-merlin
+ln -sfn "$merlin_dir" .for-jane-street-merlin/dev
+ln -sfn "$merlin_dir" .for-jane-street-merlin/prod
+
+echo "$PWD/.for-jane-street-merlin" > .merlin-binaries
+cp .local-ocaml-lib .ocaml-lib

jenga.conf

@@ -0,0 +1 @@
+Stub for Jane Street .merlin-binaries search