build with system dependencies if possible
build with system dependencies if possible

Summary:
Watchman compilation is much faster when system packages can be
used. Even though the Vagrant VMs don't benefit from the .deb and .rpm
lists in our getdeps manifests, enable the feature now.

Reviewed By: genevievehelsel

Differential Revision: D36486987

fbshipit-source-id: 1666a77a5283a2dbb60db9fa65e6ce1d332533ad

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