cmake: don't add CMAKE_INSTALL_PREFIX for mroonga bundled build
cmake: don't add CMAKE_INSTALL_PREFIX for mroonga bundled build

Because CPack packaging process fails.

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