Make some interactive mode test case improvements.

Make some interactive mode test case improvements.

* testsuite/systemtap.pass1-4/buildok-interactive.exp: Only run the test
  if stap was compiled with the readline library. Add support for the
  "compatible_version" option.
* session.cxx (systemtap_session::version): Report whether stap was
  compiled with the readline library.
* testsuite/lib/systemtap.exp (readline_p): Add a new function that
  reports if systemtap was compiled with the readline library.