fix -Wno- compiler flag detection.
fix -Wno- compiler flag detection.

This change is based on a patch by Arnaud Fontaine.

Reported by Kazuhiko. Thanks!!!

naoa/mroonga,naoa/mroonga,yoku0825/mroonga,yoku0825/mroonga,mroonga/mroonga,naoa/mroonga,yoku0825/mroonga,yoku0825/mroonga,naoa/mroonga,yoku0825/mroonga,yoku0825/mroonga,naoa/mroonga,mroonga/mroonga,mroonga/mroonga,naoa/mroonga,mroonga/mroonga