Makefile: always (re)set DC_SHA1 on fallback

Makefile: always (re)set DC_SHA1 on fallback

Fix an edge case introduced in in e6b07da2780 (Makefile: make DC_SHA1
the default, 2017-03-17), when DC_SHA1 was made the default fallback
we started unconditionally adding to BASIC_CFLAGS and LIB_OBJS, so
we'd use the sha1collisiondetection by default.

But the "DC_SHA1" variable remained unset, so e.g.:

	make test DC_SHA1= T=t0013*.sh

Would skip the sha1collisiondetection tests, as we'd write
"DC_SHA1=''" to "GIT-BUILD-OPTIONS", but if we manually removed that
test prerequisite we'd pass the test (which we couldn't if we weren't
using sha1collisiondetection).

So let's have the fallback assignment use the 'override' directive
instead of the ":=" simply expanded variable introduced in
e6b07da2780. In this case we explicitly want to override the user's
choice.

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