Handle corrupt Systeml.sig when moving build from Moscow ML to Poly.

This is analogous to what was done in b151b8f (Jul 2009), which coped
with building a Moscow ML setup on top of a build that used to be
Poly. There the claim was made that the reverse situation (the one
fixed in this commit) didn't need anything done to it. That was
because poly's Systeml.ui was being generated afresh with every
configure, right up until 331d321 (Jan 2014).

tools-poly/configure.sml

@@ -260,14 +260,21 @@ let
   val sigfile = fullPath [holdir, "tools", "Holmake", "Systeml.sig"]
   val uifile = fullPath [holdir, "sigobj", "Systeml.ui"]
   fun to_sigobj s = bincopy s (fullPath [holdir, "sigobj", Path.file s])
+  val uifile_content =
+      fullPath [holdir, "sigobj", "Systeml.sig"] ^ "\n"
 in
   if not (canread uifile) orelse
-     Time.>(modTime sigfile, modTime uifile)
+     Time.>(modTime sigfile, modTime uifile) orelse
+     OS.FileSys.fileSize uifile > size uifile_content
+     (* if the file is this large it's been generated by Moscow ML, or is
+        otherwise wrong *)
   then
     let
       val oo = TextIO.openOut uifile
     in
-      TextIO.output (oo, fullPath [holdir, "sigobj", "Systeml.sig"] ^ "\n");
+      (* note how this is "compiling" straight into sigobj, rather than
+         doing anything in the source directory, tools/Holmake *)
+      TextIO.output (oo, uifile_content);
       TextIO.closeOut oo;
       print "sig "
     end