Makefile: move the setting of *FLAGS closer to "include"

Makefile: move the setting of *FLAGS closer to "include"

Move the setting of variables like CFLAGS down past settings like
"prefix" and default programs like "TAR" to just before we do the
include from "config.mak.*".

There's no functional changes here yet, but move note that
"ALL_CFLAGS" and "ALL_LDFLAGS" are moved below the include. A
follow-up change will tweak those depending on a variable set in
config.mak.dev.

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