Makefile: don't error out under DC_SHA1_EXTERNAL if DC_SHA1_SUBMODULE=auto

Fix a logic error in the initial introduction of DC_SHA1_EXTERNAL. If git.git has a sha1collisiondetection submodule checked out the logic to set DC_SHA1_SUBMODULE=auto would interact badly with the check for whether DC_SHA1_SUBMODULE was set. It would error out, meaning that there's no way to build git with DC_SHA1_EXTERNAL=YesPlease without deinit-ing the submodule. Instead, adjust the logic to only fire if the variable is to something else than "auto" which would mean it's a mistake on the part of whoever's building git, not just the Makefile tripping over its own logic. 1. 3964cbbb5c ("sha1dc: allow building with the external sha1dc library", 2017-08-15) 2. cac87dc01d ("sha1collisiondetection: automatically enable when submodule is populated", 2017-07-01) Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>