Check for the readline library before compiling in interactive mode.

Check for the readline library before compiling in interactive mode.

* configure.ac: Check for the readline library and any other libraries it
  needs.
* Makefile.am: If HAVE_LIBREADLINE is set, compile interactive.cxx and
  link with the needed readline libraries.
* main.cxx: Only call 'interactive_mode()' if we have the readline
  library.
* session.cxx: Only show help for interactive mode if we've got the
  readline library.
* Makefile.in: Regenerated.
* config.in: Ditto.
* configure: Ditto.