Makefile: Fix Makefiles to depend on all child Makefile fragments.

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.