Makefile: add support for Apple CommonCrypto facility

As of Mac OS X 10.7, Apple deprecated all OpenSSL functions due to
OpenSSL ABI instability, thus leading to build warnings.  As a
replacement, Apple encourages developers to migrate to its own (stable)
CommonCrypto facility.

Introduce boilerplate which controls whether Apple's CommonCrypto
facility is employed (enabled by default).  Also add a
NO_APPLE_COMMON_CRYPTO build flag with which the user can opt out to
use OpenSSL instead.

[es: extracted CommonCrypto-related Makefile boilerplate into separate
introductory patch]

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>

Makefile

@@ -137,6 +137,10 @@ all::
 # specify your own (or DarwinPort's) include directories and
 # library directories by defining CFLAGS and LDFLAGS appropriately.
 #
+# Define NO_APPLE_COMMON_CRYPTO if you are building on Darwin/Mac OS X
+# and do not want to use Apple's CommonCrypto library.  This allows you
+# to provide your own OpenSSL library, for example from MacPorts.
+#
 # Define BLK_SHA1 environment variable to make use of the bundled
 # optimized C SHA1 routine.
 #
@@ -1054,6 +1058,10 @@ ifeq ($(uname_S),Darwin)
 			BASIC_LDFLAGS += -L/opt/local/lib
 		endif
 	endif
+	ifndef NO_APPLE_COMMON_CRYPTO
+		APPLE_COMMON_CRYPTO = YesPlease
+		COMPAT_CFLAGS += -DAPPLE_COMMON_CRYPTO
+	endif
 	NO_REGEX = YesPlease
 	PTHREAD_LIBS =
 endif