[PATCH v3 3/3] sha1collisiondetection: automatically enable when submodule is populated

["Ævar Arnfjörð Bjarmason" <avarab@gmail.com>]

From: Junio C Hamano <gitster@pobox.com>

If a user wants to experiment with the version of collision
detecting sha1 from the submodule, the user needed to not just
populate the submodule but also needed to turn the knob.

A Makefile trick is easy enough to do so, so let's do this.  When
somebody with a copy of the submodule populated wants not to use it,
that can be done by overriding it in config.mak or from the command
line.

Signed-off-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
---
 Makefile | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/Makefile b/Makefile
index f0cac1f246..3baa1eac0b 100644
--- a/Makefile
+++ b/Makefile
@@ -1009,6 +1009,10 @@ EXTLIBS =
 
 GIT_USER_AGENT = git/$(GIT_VERSION)
 
+ifeq ($(wildcard sha1collisiondetection/lib/sha1.h),sha1collisiondetection/lib/sha1.h)
+DC_SHA1_SUBMODULE = auto
+endif
+
 include config.mak.uname
 -include config.mak.autogen
 -include config.mak
-- 
2.13.1.611.g7e3b11ae1