We add a magic line to the beginning of each Makefile.local file to
help the editor know that it should use makefile mode for editing the
file, (even though the filename isn't exactly "Makefile").
Edited-by: Carl Worth <cworth@cworth.org>: Expand treatment from
emacs/Makefile.local to each instance of Makefile.local.
We were previously maintaining two lists of the child Makefile
fragments---one for the includes and another for the dependencies. So,
of course, they drifted and the dependency list wasn't up to date.
We fix this by adding a single subdirs variable, and then using GNU
Makefile substitution to generate both the include and the dependency
lists.
Some side effect of this change caused the '=' assignment of the dir
variable to not work anymore. I'm not sure why that is, but using ':='
makes sense here and fixes the problem.