cmake: remove needless "${" and "}"
cmake: remove needless "${" and "}"

We should not use "${" and "}" in if() for checking whether the
variable has valid value.

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