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configure.ac: link Foundation framework

• configure.ac (XLDFLAGS): link against Foundation framework and
  let __NSPlaceholderDictionary initialize, to get rid of crash
  after fork on macOS High Sierra. [ruby-core:83239] [Bug #14009]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@60182 b2dd03c8-39d4-4d8f-98ff-823fe69b080e