cache.h: eliminate SHA-1 deprecation warnings on Mac OS X

cache.h: eliminate SHA-1 deprecation warnings on Mac OS X

As of Mac OS X 10.7, Apple deprecated all OpenSSL functions due to
OpenSSL ABI instability, thus leading to build diagnostics such as:

	warning: 'SHA1_Init' is deprecated
	(declared at /usr/include/openssl/sha.h:121)

Silence the warnings by using Apple's CommonCrypto SHA-1 replacement
functions for SHA1_Init(), SHA1_Update(), and SHA1_Final().

COMMON_DIGEST_FOR_OPENSSL is defined to instruct
<CommonCrypto/CommonDigest.h> to provide compatibility macros
associating OpenSSL SHA-1 functions with their CommonCrypto
counterparts.

[es: reworded commit message]

Signed-off-by: David Aguilar <davvid@gmail.com>
Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>