message: Use a backwards-compatible way of retreiving the host os version.
  It looks like the sysctl was introduced in 10.13.
  <rdar://problem/40419341>
diff: // REQUIRES: darwin
  // RUN: echo -n "@available(macOS " >%t.swift
  -// RUN: sysctl kern.osproductversion | awk '{printf $2}' >>%t.swift
  +// RUN: python -c 'from __future__ import print_function; import platform; print(platform.mac_ver()[0],end="")' >>%t.swift
  // RUN: echo ", *) func newAPI() -> Int { return 11+12 }" >>%t.swift
  // RUN: echo "newAPI()" >>%t.swift
  