Parallelise building dynlink_compilerlibs

This performs two weird tricks that together knock ~30 seconds off the build
time in dev mode and ~40 seconds outside dev mode:

- To build `dynlink_compilerlibs`, copy both `parser.ml` and `parser.mli` from
  the main compiler source. Copying just `parser.ml` means that the native build
  depends on the .cmi produced by the bytecode build.

- Explicitly add the .cma and .cmxa files for `dynlink` and
  `flambda2_from_lambda` to the `install` alias. It's not clear why this helps,
  but it appears that Dune is being excessively lazy in discovering installable
  targets, waiting until `ocamlcommon` is built before it even considers any
  targets in these libraries, including those that don't depend on `ocamlcommon`
  (and take quite a while to build). Redundantly adding targets to `install`
  seems to be the nudge Dune needs.

dune

@@ -866,3 +866,20 @@
  (package ocaml)
  (files
   (include compiler-libs-installation.sexp)))
+
+; This is entirely redundant, but without it, each target in these libraries
+; and their dependencies waits until ocamlcommon is built, including those
+; targets that don't actually depend on it. This produces severe bottlenecking,
+; costing about 15 seconds when building in dev mode. It's not immediately clear
+; why Dune fails to parallelise here. Going by the progress indicator, my best
+; guess is that targets in subdirectories aren't even discovered until
+; ocamlcommon is built, unless something in the subdirectory is explicitly made
+; a dependency.
+(alias
+ (name install)
+ (package ocaml)
+ (deps
+  otherlibs/dynlink/dynlink.cma
+  otherlibs/dynlink/dynlink.cmxa
+  middle_end/flambda2/from_lambda/flambda2_from_lambda.cma
+  middle_end/flambda2/from_lambda/flambda2_from_lambda.cmxa))

otherlibs/dynlink/dune

@@ -42,14 +42,21 @@
  (targets parser.ml)
  (deps ../../parser.ml)
  (action
-  (bash
-   "echo \"module MenhirLib = CamlinternalMenhirLib\" > parser.ml && cat ../../parser.ml >> parser.ml")))
+  (with-stdout-to %{targets}
+   (progn
+    (echo "module MenhirLib = CamlinternalMenhirLib")
+    (cat %{deps})))))
 
 (rule
- (targets camlinternalMenhirLib.ml)
- (deps ../../parsing/camlinternalMenhirLib.ml)
+ (targets parser.mli)
+ (deps ../../parser.mli)
  (action
-  (copy %{deps} %{targets})))
+  (with-stdout-to %{targets}
+   (progn
+    (echo "module MenhirLib = CamlinternalMenhirLib")
+    (cat %{deps})))))
+
+(copy_files ../../parsing/camlinternalMenhirLib.ml{,i})
 
 (library
  (name dynlink_compilerlibs)
@@ -59,8 +66,6 @@
   (:standard
    -strict-sequence
    -absname
-   -w
-   +67
    -bin-annot
    -safe-string
    -strict-formats))