ocaml: enable parallel building

Enable parallel building for ocaml-4.08 and above. tested as:

    $ nix build -f. ocaml-ng.ocamlPackages_{4_{00_1,01_0,02,03,04,05,06,07,08,09,10,11,12,13},latest}.ocaml --keep-going

ocaml build system supports parallel building, but but for multiple
top-level targets at the same time as it usually spawns subprocess
$(MAKE) that occasionally conflict with one another. To work it around
we use tiny Makefile with a single rule that calls top-level targets
sequentially as makefile calls:

    nixpkgs_world_bootstrap_world_opt:
       $(MAKE) world
       $(MAKE) bootstrap
       $(MAKE) world.opt

On a 16-core machine ocaml-4.12 build speeds up from 6m55s to 1m35s.

Releases 4_00_1, 4_01_0, 4_04 and 4_05 still have some race in them.
Thus this change enables parallel builds only for ocaml-4.06 and above.

Adapted from #142723

upstreams's CI tests the parallel makefile: https://github.com/ocaml/ocaml/issues/10235#issuecomment-782100584
The limit was chosen to be 4.08 because it was released in 2019, not too
long before the above link.

pkgs/development/compilers/ocaml/Makefile.nixpkgs

@@ -0,0 +1,16 @@
+# ocaml build system does not allow for parallel building of some
+# top-level targets like 'world', 'bootstrap', 'world.opt' as
+# then spawn '$(MAKE) all' subprocesses that conflict among each
+# other. But we would still like to run each target in parallel
+# individually. This file defines such entry points.
+
+# Re-export all existing phases to make 'make install' work as is.
+include Makefile
+
+nixpkgs_world:
+	$(MAKE) world
+
+nixpkgs_world_bootstrap_world_opt:
+	$(MAKE) world
+	$(MAKE) bootstrap
+	$(MAKE) world.opt

pkgs/development/compilers/ocaml/generic.nix

@@ -68,7 +68,18 @@ stdenv.mkDerivation (args // {
   # x86_64-unknown-linux-musl-ld: -r and -pie may not be used together
   hardeningDisable = lib.optional (lib.versionAtLeast version "4.09" && stdenv.hostPlatform.isMusl) "pie";
 
-  buildFlags = [ "world" ] ++ optionals useNativeCompilers [ "bootstrap" "world.opt" ];
+  # Older versions have some race:
+  #  cp: cannot stat 'boot/ocamlrun': No such file or directory
+  #  make[2]: *** [Makefile:199: backup] Error 1
+  enableParallelBuilding = lib.versionAtLeast version "4.08";
+
+  # Workaround lack of parallelism support among top-level targets:
+  # we place nixpkgs-specific targets to a separate file and set
+  # sequential order among them as a single rule.
+  makefile = ./Makefile.nixpkgs;
+  buildFlags = if useNativeCompilers
+    then ["nixpkgs_world_bootstrap_world_opt"]
+    else ["nixpkgs_world"];
   buildInputs = optional (!lib.versionAtLeast version "4.07") ncurses
     ++ optionals useX11 [ libX11 xorgproto ];
   propagatedBuildInputs = optional spaceTimeSupport libunwind;