git-compat-util: suppress unavoidable Apple-specific deprecation warnings

git-compat-util: suppress unavoidable Apple-specific deprecation warnings

With the release of Mac OS X 10.7 in July 2011, Apple deprecated all
openssl.h functionality due to OpenSSL ABI (application binary
interface) instability, resulting in an explosion of compilation
warnings about deprecated SSL, SHA1, and X509 functions (among others).

61067954ce (cache.h: eliminate SHA-1 deprecation warnings on Mac OS X;
2013-05-19) and be4c828b76 (imap-send: eliminate HMAC deprecation
warnings on Mac OS X; 2013-05-19) attempted to ameliorate the situation
by taking advantage of drop-in replacement functionality provided by
Apple's (ABI-stable) CommonCrypto facility, however CommonCrypto
supplies only a subset of deprecated OpenSSL functionality, thus a host
of warnings remain.

Despite this shortcoming, it was hoped that Apple would ultimately
provide CommonCrypto replacements for all deprecated OpenSSL
functionality, and that the effort started by 61067954ce and be4c828b76
would be continued and eventually eliminate all deprecation warnings.
However, now 3.5 years later, and with Mac OS X at 10.10, the hoped-for
CommonCrypto replacements have not yet materialized, nor is there any
indication that they will be forthcoming.

These Apple-specific warnings are pure noise: they don't tell us
anything useful and we have no control over them, nor is Apple likely to
provide replacements any time soon. Such noise may obscure other
legitimate warnings, therefore silence them.

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