Changes from 2.21.0 to 2.21.1

• Fixed a regression introduced in PR #682 causing incompatibility
  at least with Fedora and Gentoo. Fixed in PR #690. Thanks to
  Michał Górny (@mgorny) for reporting and Teodor Tsirpanis
  (@teo-tsirpanis) for the actual fix.

• Restrict overmatching MACH ifdef to only trigger on OSX and Mach.
  Thanks to Maximilian Downey Twiss (@Zopolis4) for the PR (#688).