allow building watchman on macOS
allow building watchman on macOS

Summary: For a faster iteration flow, reenable the Buck build of Watchman on macOS.

Reviewed By: wez

Differential Revision: D19737251

fbshipit-source-id: 9ccbc8b71c7986d16e1aeb6b61dd595d47b9250e

nodakai/watchman,facebook/watchman,facebook/watchman,facebook/watchman,facebook/watchman,nodakai/watchman,nodakai/watchman,facebook/watchman,facebook/watchman,facebook/watchman,nodakai/watchman,facebook/watchman,nodakai/watchman,nodakai/watchman,nodakai/watchman,facebook/watchman,nodakai/watchman,nodakai/watchman