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