Double-colon rule behaviour isn't really compatible with parallel
build. This gets rid of all of them, so we can codify the
directory dependencies.

This leaves just three problems, which aren't really fixable with
the current build system without completely replacing it:

* everything depends on nsinstall
* everything depends on installed headers
* ckfw child directories depend on the build parent libs

This is handled by the prepare_build target.

Overall this allows most if the build to run in parallel.

P.S. the release_md:: has to stay :-(
P.P.S. no clue, why freebl must use libs: instead of using the
TARGETS and .PHONY variables

Differential Revision: https://phabricator.services.mozilla.com/D69023

--HG--
extra : moz-landing-system : lando