Correctly encode dependency on runtime/sak for StdlibModules

Makefile

@@ -825,16 +825,16 @@ clean::
 # The standard library
 
 .PHONY: library
-library: ocamlc | $(SAK)
+library: ocamlc
 	$(MAKE) -C stdlib $(BOOT_FLEXLINK_CMD) all
 
 .PHONY: library-cross
-library-cross: | $(SAK)
+library-cross:
 	$(MAKE) -C stdlib \
 	  $(BOOT_FLEXLINK_CMD) OCAMLRUN=../runtime/ocamlrun$(EXE) all
 
 .PHONY: libraryopt
-libraryopt: | $(SAK)
+libraryopt:
 	$(MAKE) -C stdlib $(BOOT_FLEXLINK_CMD) allopt
 
 partialclean::

Makefile.common

@@ -162,3 +162,11 @@ OCAMLYACCFLAGS ?=
 	$(OCAMLYACC) $(OCAMLYACCFLAGS) $<
 
 SAK = $(ROOTDIR)/runtime/sak$(EXE)
+
+# stdlib/StdlibModules cannot be include'd unless $(SAK) has been built. These
+# two rules add that dependency. They have to be pattern rules since
+# Makefile.common is included before default targets.
+$(ROOTDIR)/%/sak$(EXE):
+	$(MAKE) -C $(ROOTDIR)/$* sak$(EXE)
+
+$(ROOTDIR)/%/StdlibModules: $(SAK) ;