Makefile: allow for combining DEVELOPER=1 and CFLAGS="..."

Ever since the DEVELOPER=1 facility introduced there's been no way to
have custom CFLAGS (e.g. CFLAGS="-O0 -g -ggdb3") while still
benefiting from the set of warnings and assertions DEVELOPER=1
enables.

This is because the semantics of variables in the Makefile are such
that the user setting CFLAGS overrides anything we set, including what
we're doing in config.mak.dev[1].

So let's introduce a "DEVELOPER_CFLAGS" variable in config.mak.dev and
add it to ALL_CFLAGS. Before this the ALL_CFLAGS variable
would (basically, there's some nuance we won't go into) be set to:

    $(CPPFLAGS) [$(CFLAGS) *or* $(CFLAGS) in config.mak.dev] $(BASIC_CFLAGS) $(EXTRA_CPPFLAGS)

But will now be:

    $(DEVELOPER_CFLAGS) $(CPPFLAGS) $(CFLAGS) $(BASIC_CFLAGS) $(EXTRA_CPPFLAGS)

The reason for putting DEVELOPER_CFLAGS first is to allow for
selectively overriding something DEVELOPER=1 brings in. On both GCC
and Clang later settings override earlier ones. E.g. "-Wextra
-Wno-extra" will enable no "extra" warnings, but not if those two
arguments are reversed.

Examples of things that weren't possible before, but are now:

    # Use -O0 instead of -O2 for less painful debuggng
    DEVELOPER=1 CFLAGS="-O0 -g"
    # DEVELOPER=1 plus -Wextra, but disable some of the warnings
    DEVELOPER=1 DEVOPTS="no-error extra-all" CFLAGS="-O0 -g -Wno-unused-parameter"

The reason for the patches leading up to this one re-arranged the
various *FLAGS assignments and includes is just for readability. The
Makefile supports assignments out of order, e.g.:

    $ cat Makefile
    X = $(A) $(B) $(C)
    A = A
    B = B
    include c.mak
    all:
            @echo $(X)
    $ cat c.mak
    C=C
    $ make
    A B C

So we could have gotten away with the much smaller change of changing
"CFLAGS" in config.mak.dev to "DEVELOPER_CFLAGS" and adding that to
ALL_CFLAGS earlier in the Makefile "before" the config.mak.*
includes. But I think it's more readable to use variables "after"
they're included.

1. https://www.gnu.org/software/make/manual/html_node/Overriding.html

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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