We were previously using separate CFLAGS and NOTMUCH_CFLAGS variables
in an attempt to allow the user to specify CFLAGS on the command-line.
However, that's just a lot of extra noise in the Makefile when we can
instead let the user specify what is desired for CFLAGS and then use
an override to append the things we require. So our Makefile is much
neater now.
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.
