Makefile: move "strip" assignment down from flags

Makefile: move "strip" assignment down from flags

Move the assignment of the "STRIP" variable down to where we're
setting variables with the names of other programs.

For consistency with those use "=" for the assignment instead of
"?=". I can't imagine why this would need to be different than the
rest, and 4dc00021f7 ("Makefile: add 'strip' target", 2006-01-12)
which added it doesn't provide an explanation.

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