- fix generation of sysnum.h for parallel builds, second take.

  Revert r21503 and redo more cleanly.

Makerules

@@ -199,14 +199,14 @@ $(interp): $(top_builddir)lib/interp.c
 	$(compile.c)
 	$(Q)$(STRIPTOOL) -x -R .note -R .comment $@
 
-$(ldso): | $(sub_headers)
+$(ldso):
 	@cd $(top_builddir); $(MAKE) lib/$(patsubst %.$(MAJOR_VERSION),%,$(notdir $@))
 
 $(libc):
 	@cd $(top_builddir); $(MAKE) lib/$(patsubst %.$(MAJOR_VERSION),%,$(notdir $@))
 
 $(headers_dep):
-	$(Q)cd $(top_builddir); $(MAKE) headers
+	$(Q)cd $(top_builddir); $(MAKE) pregen
 
 CRT := crt1