cmake: support embedded Groonga with Clang
cmake: support embedded Groonga with Clang

MDEV: 7293

Reported by Bernard Spil. Thanks!!!

groonga/groonga,redfigure/groonga,naoa/groonga,cosmo0920/groonga,komainu8/groonga,naoa/groonga,redfigure/groonga,kenhys/groonga,komainu8/groonga,redfigure/groonga,kenhys/groonga,kenhys/groonga,hiroyuki-sato/groonga,redfigure/groonga,groonga/groonga,hiroyuki-sato/groonga,komainu8/groonga,cosmo0920/groonga,komainu8/groonga,hiroyuki-sato/groonga,komainu8/groonga,kenhys/groonga,naoa/groonga,groonga/groonga,groonga/groonga,komainu8/groonga,komainu8/groonga,hiroyuki-sato/groonga,komainu8/groonga,kenhys/groonga,redfigure/groonga,cosmo0920/groonga,naoa/groonga,cosmo0920/groonga,kenhys/groonga,groonga/groonga,redfigure/groonga,cosmo0920/groonga,cosmo0920/groonga,naoa/groonga,hiroyuki-sato/groonga,groonga/groonga,groonga/groonga,naoa/groonga,naoa/groonga,hiroyuki-sato/groonga,redfigure/groonga,kenhys/groonga,hiroyuki-sato/groonga,groonga/groonga,cosmo0920/groonga,cosmo0920/groonga,hiroyuki-sato/groonga,redfigure/groonga,naoa/groonga,kenhys/groonga