Previously, the top-level Makefile was explicitly adding -I./lib to
the compiler flags. However, that's something that's much better done
from within the Makefile.local fragment within the lib directory
itself.
I saw this recommendation in the implementation notes for "Recursive
Make Considered Harmful" and then the further recommendation for
implementing the idea in the GNU make manual.
The idea is that if any of the files change then we need to regenerate
the dependency file before we regenerate any targets.
The approach from the GNU make manual is simpler in that it just uses
a sed script to fix up the output of an extra invocation of the
compiler, (as opposed to the approach in the implementation notes from
the paper's author which use a wrapper script for the compiler that's
always invoked rather than the compiler itself).
The idea here is that every Makefile at each lower level will be an
identical, tiny file that simply defers to a top-level make.
Meanwhile, the Makefile.local file at each level is a Makefile snippet
to be included at the top-level into a large, flat Makefile. As such,
it needs to define its rules with the entire relative directory to
each file, (typically in $(dir)). The local files can also append to
variables such as SRCS and CLEAN for files to be analyzed for
dependencies and to be cleaned.
