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>