cmake: don't use version-gen.sh on no /bin/sh environment
cmake: don't use version-gen.sh on no /bin/sh environment

For example, Windows.

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