CMake: fall back to OpenSSL on older OS X

Starting at OS X 10.8, the Security framework offers some functions
which are unified across OS X and iOS. These are the functions that we
use.

Older versions of OS X do not have these functions and we fail to
compile. In these situations, fall back to using OpenSSL for our TLS
stream instead.

CMakeLists.txt

@@ -152,8 +152,18 @@ STRING(REGEX REPLACE "^.*LIBGIT2_SOVERSION ([0-9]+)$" "\\1" LIBGIT2_SOVERSION "$
 INCLUDE_DIRECTORIES(src include)
 
 IF (SECURITY_FOUND)
+  # OS X 10.7 and older do not have some functions we use, fall back to OpenSSL there
+  CHECK_LIBRARY_EXISTS("${SECURITY_DIRS}" SSLCreateContext "Security/SecureTransport.h" HAVE_NEWER_SECURITY)
+  IF (HAVE_NEWER_SECURITY)
     MESSAGE("-- Found Security ${SECURITY_DIRS}")
     LIST(APPEND LIBGIT2_PC_LIBS "-framework Security")
+  ELSE()
+    MESSAGE("-- Security framework is too old, falling back to OpenSSL")
+    SET(SECURITY_FOUND "NO")
+    SET(SECURITY_DIRS "")
+    SET(SECURITY_DIR "")
+    SET(USE_OPENSSL "ON")
+  ENDIF()
 ENDIF()
 
 IF (COREFOUNDATION_FOUND)