am b8ebbcc8: am 90b06ac7: only use faster SHA-1 code on machines with good libc

Merge commit 'b8ebbcc88cdab8b0c8d96feca4507dcae11607f4'

* commit 'b8ebbcc88cdab8b0c8d96feca4507dcae11607f4':
  only use faster SHA-1 code on machines with good libc