A new configure option --with-devtools (which sets MOZ_DEVTOOLS) is added to
control whether all DevTools, just the server, or no DevTools are included.
This defaults to just the server.
Applications should also include /devtools within their moz.build tree, so that
DIST_SUBDIR is in effect for all DevTools files if it is used by the app.
The configure option has explicitly thrown an error for more than a year now,
and it happens that the remaining way to still forcefully use it has been
broken for more than 8 months.
