cmake: Add CMake check for libssh2 memory credential passing support

CMakeLists.txt

@@ -270,6 +270,11 @@ IF (LIBSSH2_FOUND)
 	LINK_DIRECTORIES(${LIBSSH2_LIBRARY_DIRS})
 	SET(LIBGIT2_PC_REQUIRES "${LIBGIT2_PC_REQUIRES} libssh2")
 	SET(SSH_LIBRARIES ${LIBSSH2_LIBRARIES})
+
+	CHECK_LIBRARY_EXISTS(${LIBSSH2_LIBRARIES} libssh2_userauth_publickey_frommemory "" HAVE_LIBSSH2_MEMORY_CREDENTIALS)
+	IF (HAVE_LIBSSH2_MEMORY_CREDENTIALS)
+		ADD_DEFINITIONS(-DGIT_SSH_MEMORY_CREDENTIALS)
+	ENDIF()
 ELSE()
 	MESSAGE(STATUS "LIBSSH2 not found. Set CMAKE_PREFIX_PATH if it is installed outside of the default search path.")
 ENDIF()