message: _build: macos: update code looking for Python
  Make it equal to the one used in memo.
diff: @@ -15,17 +15,18 @@ import drake
  import drake.cxx
  import drake.cxx.boost
  +def find_python3():
  python3 = None
  for minor in range(9, 0, -1):
  - for dir in ['/usr/local', '/opt/local/Library']:
  + # /opt/local is MacPorts.
  + # /usr/local/Cellar/ is Brew.
  + for dir in ['/usr/local', '/opt/local/Library', '/usr/local/Cellar/python3/3.5.1']:
  p = '{}/Frameworks/Python.framework/Versions/3.{}'.format(dir, minor)
  if os.path.isdir(p):
  - python3 = p
  - break
  - # Guido says we don't need multi-level breaks.
  - if python3:
  - break
  -print("using python3 =", python3)
  + return p
  +
  +python3 = find_python3()
  +print("using python3 =", python3, file=sys.stderr)
  with drake.Drake('../..') as d:
  cxx_compiler = os.getenv('CXX', 'clang++')
  