add option not to prefer /usr/bin python on mac

Reviewed By: chadaustin

Differential Revision: D29084022

fbshipit-source-id: 0605c1bfdd86ab94f4aa6893737b296ab4cdd913

Author: genevievehelsel

build/fbcode_builder/manifests/watchman

@@ -38,6 +38,7 @@ ENABLE_EDEN_SUPPORT=ON
 WATCHMAN_STATE_DIR=/opt/facebook/watchman/var/run/watchman
 # tell cmake not to try to create /opt/facebook/...
 INSTALL_WATCHMAN_STATE_DIR=OFF
+USE_SYS_PYTHON=OFF
 
 [depends.environment]
 WATCHMAN_VERSION_OVERRIDE