message: configure.ac: Print error when libboost_system is not detected
  This is to replace a quite misleading error about
  "No working boost sleep implementation found."
  further down in the configure.ac script from obscuring this issue.
diff: @@ -671,6 +671,10 @@ define(MINIMUM_REQUIRED_BOOST, 1.47.0)
  dnl Check for boost libs
  AX_BOOST_BASE([MINIMUM_REQUIRED_BOOST])
  AX_BOOST_SYSTEM
  +if test x$BOOST_SYSTEM_LIB = x; then
  + AC_MSG_ERROR([Boost system library required.])
  +fi
  +
  AX_BOOST_FILESYSTEM
  AX_BOOST_PROGRAM_OPTIONS
  AX_BOOST_THREAD
  