Merge branch 'ab/sha1dc-build'

Push the submodule version of collision-detecting SHA-1 hash implementation a bit harder on builders. * ab/sha1dc-build: sha1dc_git.h: re-arrange an ifdef chain for a subsequent change Makefile: under "make dist", include the sha1collisiondetection submodule Makefile: don't error out under DC_SHA1_EXTERNAL if DC_SHA1_SUBMODULE=auto