Respect MAKE_JOBS during bootstrap phase
Index: Makefile
--- Makefile.orig
+++ Makefile
@@ -47,7 +47,7 @@ release: $(BIN)
 	@$(BIN) build @install -p dune --profile dune-bootstrap
 
 $(BIN):
-	@ocaml boot/bootstrap.ml
+	@ocaml boot/bootstrap.ml -j $(JOBS)
 
 dev: $(BIN)
 	$(BIN) build @install