* use the same HAVE_C/CXX_ variables for compiler flag tests as the rest of
  the server and tokudb - to use cached results
* plugin's name should be "mroonga" not "ha_mroonga"
* don't use set_property(TARGET plugin_name ...), it aborts cmake when a plugin
  id disabled, because the target doesn't exists in that case

result: mroonga can now be disabled from cmake command line