Makefile: create and use sections for "define" flag listing

Makefile: create and use sections for "define" flag listing

Since the "Define ..." template of comments at the top of the Makefile
was started in 5bdac8b3269 ([PATCH] Improve the compilation-time
settings interface, 2005-07-29) we've had a lot more flags added,
including flags that come in "groups". Not having any obvious
structure to the >500 line comment at the top of the Makefile has made
it hard to follow.

This change is almost entirely a move-only change, the two paragraphs
at the start of the first two sections are new, and so are the added
sections themselves, but other than that no lines are changed, only
moved.

We now list Makefile-only flags at the start, followed by stand-alone
flags, and then cover "optional library" flags in their respective
groups, followed by SHA-1 and SHA-256 flags, and finally
DEVELOPER-specific flags.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Taylor Blau <me@ttaylorr.com>