cmake: fix a bug that compiler flag checks are skipped
cmake: fix a bug that compiler flag checks are skipped

GitHub: fixes #21

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