add info to install libiberty-dev on Ubuntu; refs #9

configure.in

@@ -211,6 +211,7 @@ if test "$have_stacktrace" = "yes"; then
     - is one of them installed?
        - If yes, try: "nm <filename> | grep cplus_demangle"
           -> is the symbol defined?
+    - on Ubuntu (trusty) install libiberty-dev
  - Try to compile with option --disable-stacktrace
    If you are not a developer, you perhaps don't need it...?])])
   if test "$have_dynamicstack" != "yes"; then