cmake: prep for running with getdeps.py
cmake: prep for running with getdeps.py

Summary:
have cmake search the external dir that is maintained by getdeps.py

Refs: https://github.com/facebook/watchman/pull/636

Reviewed By: simpkins

Differential Revision: D13486487

fbshipit-source-id: fe7dea3e9652af425291012cb0e7b9a6066cfa81

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