[PATCH v3 8/9] Makefile: document default SHA-1 backend on OSX

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

Since [1] the default SHA-1 backend on OSX has been
APPLE_COMMON_CRYPTO. Per [2] we'll skip using it on anything older
than Mac OS X 10.4 "Tiger"[3].

When "DC_SHA1" was made the default in [4] this interaction between it
and APPLE_COMMON_CRYPTO seems to have been missed in. Ever since
DC_SHA1 was "made the default" we've still used Apple's CommonCrypto
instead of sha1collisiondetection on modern versions of Darwin and
OSX.

1. 61067954ce1 (cache.h: eliminate SHA-1 deprecation warnings on Mac
   OS X, 2013-05-19)
2. 9c7a0beee09 (config.mak.uname: set NO_APPLE_COMMON_CRYPTO on older
   systems, 2014-08-15)
3. We could probably drop "NO_APPLE_COMMON_CRYPTO", as nobody's likely
   to care about such on old version of OSX anymore. But let's leave that
   for now.
4. e6b07da2780 (Makefile: make DC_SHA1 the default, 2017-03-17)

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
---
 Makefile | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/Makefile b/Makefile
index 8a9f3e561f8..0ca8a9eb318 100644
--- a/Makefile
+++ b/Makefile
@@ -519,6 +519,11 @@ include shared.mak
 # Define BLK_SHA1 to make use of optimized C SHA-1 routines bundled
 # with git (in the block-sha1/ directory).
 #
+# Define NO_APPLE_COMMON_CRYPTO on OSX to opt-out of using the
+# "APPLE_COMMON_CRYPTO" backend for SHA-1, which is currently the
+# default on that OS. We'll define NO_APPLE_COMMON_CRYPTO on Mac OS
+# 10.4 or older ("Tiger", released in early 2005).
+#
 # === SHA-256 backend ===
 #
 # ==== Default SHA-256 backend ====
-- 
2.38.0.1178.g509f5fa8ce0