don't run js_of_ocaml when flow_parser hasn't changed

Summary:
`make -C src/parser js` runs ocamlbuild to build flow_parser.js's bytecode, then calls js_of_ocaml. if the inputs haven't changed, then ocamlbuild doesn't `touch` flow_parser_dot_js.byte, which we can use to skip running js_of_ocaml when the inputs haven't changed.

this is similar to using the js_of_ocaml-ocamlbuild plugin, but doesn't require an ocamlbuild plugin.

Reviewed By: gabelevi

Differential Revision: D7519542

fbshipit-source-id: 8b12494bf0dec6456bdd2517ff6d26f1a2925ca3

Author: mroch

src/parser/Makefile

@@ -83,11 +83,11 @@ dist/libflowparser.zip: \
 
 js:
 	cd $(TOP); \
-	$(OCB) -pkgs js_of_ocaml $(REL_DIR)/flow_parser_dot_js.byte; \
-	js_of_ocaml --opt 3 \
-		-o $(REL_DIR)/flow_parser.js \
-		_build/$(REL_DIR)/flow_parser_dot_js.byte; \
-	rm flow_parser_dot_js.byte
+	$(OCB) -no-links -pkgs js_of_ocaml $(REL_DIR)/flow_parser_dot_js.byte; \
+	[ -e "$(REL_DIR)/flow_parser.js" -a "$(REL_DIR)/flow_parser.js" -nt "_build/$(REL_DIR)/flow_parser_dot_js.byte" ] || \
+		js_of_ocaml --opt 3 \
+			-o $(REL_DIR)/flow_parser.js \
+			_build/$(REL_DIR)/flow_parser_dot_js.byte
 
 test-js: js
 	cd $(TOP)/packages/flow-parser; npm test