Don't hardcode X11.app's path in the launchd plist.
Only install the launchd plist if we --enable-launchd.
(cherry picked from commit 6b74c535dc331d1d621b2541492a3336f69d70a2)
Leaving xpr unflattened since we want modularity to replace that with xpc (XPluginComposite) at some point
(cherry picked from commit 48e6a75fbdd0fee86e364f02ace83f20b312a2b2)
Jeremy Huddleston