[4633] Addressed comments

configure.ac

@@ -913,7 +913,7 @@ EOF
                            #endif
                            ])],
          [AC_MSG_RESULT([yes])],
-         [AC_MSG_ERROR([HMAC functions return void: the OpenSSL version should be too old, please change for >= 1.0.1])])
+         [AC_MSG_ERROR([HMAC functions return void: please use OpenSSL version 1.0.1 or later])])
     LIBS=${LIBS_SAVED}
     CPPFLAGS=${CPPFLAGS_SAVED}
 fi

src/lib/cryptolink/openssl_compat.h

@@ -0,0 +1,48 @@
+// Copyright (C) 2016 Internet Systems Consortium, Inc. ("ISC")
+//
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this
+// file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+#include <openssl/opensslv.h>
+
+#if OPENSSL_VERSION_NUMBER < 0x10100000L
+
+#ifdef KEA_HASH
+
+// EVP_MD_CTX_new() is EVP_MD_CTX_create() in OpenSSL < 1.1
+
+inline EVP_MD_CTX* EVP_MD_CTX_new() {
+    return (EVP_MD_CTX_create());
+}
+
+// EVP_MD_CTX_free(ctx) is EVP_MD_CTX_destroy(ctx) in OpenSSL < 1.1 
+
+inline void EVP_MD_CTX_free(EVP_MD_CTX* ctx) {
+    EVP_MD_CTX_destroy(ctx);
+}
+
+#endif
+
+#ifdef KEA_HMAC
+
+// HMAC_CTX_new() implementation for OpenSSL < 1.1
+
+inline HMAC_CTX* HMAC_CTX_new() {
+    HMAC_CTX* ctx = static_cast<HMAC_CTX*>(OPENSSL_malloc(sizeof(HMAC_CTX)));
+    if (ctx != 0) {
+        HMAC_CTX_init(ctx);
+    }
+    return (ctx);
+}
+
+// HMAC_CTX_free() implementation for OpenSSL < 1.1
+
+inline void HMAC_CTX_free(HMAC_CTX* ctx) {
+    HMAC_CTX_cleanup(ctx);
+    OPENSSL_free(ctx);
+}
+
+#endif
+
+#endif