Makefiles: allow the user to add to CFLAGS and Co.

[GitMensch]

Um. I don't know if there's a "standard" method for doing this? I'd think there would be, but can't say I've ever encountered it...

I just tried adding the line

CFLAGS += $(ADDED_CFLAGS)

and running make (usual flags) ADDED_CFLAGS=-DGARBAGE, and GARBAGE was suitably defined. Such a technique could be used for all makefiles, I think. It works, but I'd like to verify that we're not re-inventing a wheel here before committing changes.

(As a side issue, it'd still be nice to make the "correct" distinction between CPPFLAGS, used for the C preprocessor, and "regular" CFLAGS, in a variety of our Makefiles. Not a big deal, but it was mentioned recently, and it's hard for me to un-see it now.)

Originally posted by @Bill-Gray in #345

The documentation for autotools effectively says:
CFLAGS, LIBS, LDFLAGS, CPPFLAGS, ... all "belong to the user":

• one should not override them
• all "project owned" vars have a prefix (like PDC_)
• the "normal" ones are used in the makefiles (either side-by side with the "project owned" ones or by or adding them to those)

I think that's a good rule.

Originally posted by @GitMensch in #345

Not sure, but that may be an easy change to do before the next release, whatever it may be...

[Bill-Gray]

The documentation for autotools effectively says:

Yup, it appears to be very blunt in saying that 'user variables' should not be modified. It's less clear as to what those variables are, but the ones you mention would make the list; if we fixed those, it would at least be a good start (and we'd probably see other variables that should be fixed similarly).

The damage varies. Looking at the Makefile for WinCon, I see that we append flags to CFLAGS. Perhaps I'm missing something, but I can't see how that's actually a problem. We're respecting the user's choices for that variable, and saying "ah, we also are going to need the following flags".

If I am missing something and we must consider CFLAGS to be a constant, we could set

OUR_CFLAGS = $(CFLAGS)

and then append to OUR_CFLAGS and never use CFLAGS in the rest of the Makefile. But I'm unconvinced that we need to fix that case.

But in the same Makefile, we set LDFLAGS = -g or LDFLAGS =. We basically need to (at minimum) drop the latter and change the former to read LDFLAGS += -g.