sha1dc: allow building with the external sha1dc library
Some distros provide SHA1 collision-detect code as a shared library.
It's the same code as we have in git tree (but may be with a different
init default for hash), and git can link with it as well; at least, it
may make maintenance easier, according to our security guys.
This patch allows user to build git linking with the external sha1dc
library instead of the built-in code. User needs to define
DC_SHA1_EXTERNAL explicitly. As default without it, the built-in
sha1dc code is used like before.
Signed-off-by: Takashi Iwai <firstname.lastname@example.org>
Signed-off-by: Junio C Hamano <email@example.com>