Re-insert %.sl rule into Makefile.hpux ... seems it is not

quite as unused as I thought ... how embarrassing.

src/makefiles/Makefile.hpux

@@ -24,3 +24,8 @@ endif
 # to catch null pointer dereferences, and specify -E to make all symbols
 # visible to dynamically linked shared libraries.
 LDFLAGS+= -Wl,+b -Wl,$(LIBDIR) -Wl,-z -Wl,-E
+
+# Rule for building shared libs (currently used only for regression test
+# shlib ... should go away, since this is not really enough knowledge)
+%.sl: %.o
+	$(LD) -b -o $@ $<