imap-send: eliminate HMAC deprecation warnings on Mac OS X

As of Mac OS X 10.7, Apple deprecated all OpenSSL functions due to
OpenSSL ABI instability.  Silence the warnings by using Apple's
CommonCrypto HMAC replacement functions.

[es: reworded commit message; check APPLE_COMMON_CRYPTO instead of
abusing COMMON_DIGEST_FOR_OPENSSL]

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>

imap-send.c

@@ -29,8 +29,18 @@
 #ifdef NO_OPENSSL
 typedef void *SSL;
 #else
+#ifdef APPLE_COMMON_CRYPTO
+#include <CommonCrypto/CommonHMAC.h>
+#define HMAC_CTX CCHmacContext
+#define HMAC_Init(hmac, key, len, algo) CCHmacInit(hmac, algo, key, len)
+#define HMAC_Update CCHmacUpdate
+#define HMAC_Final(hmac, hash, ptr) CCHmacFinal(hmac, hash)
+#define HMAC_CTX_cleanup(ignore)
+#define EVP_md5() kCCHmacAlgMD5
+#else
 #include <openssl/evp.h>
 #include <openssl/hmac.h>
+#endif
 #include <openssl/x509v3.h>
 #endif