src: fix build error without OpenSSL support

PR nodejs#3890 [1] introduced the variable ALLOW_INSECURE_SERVER_DHPARAM defined
in src/node_crypto.cc. However, if nodejs is built without OpenSSL support,
the build fails:
 error: ‘ALLOW_INSECURE_SERVER_DHPARAM’ was not declared in this scope
       ALLOW_INSECURE_SERVER_DHPARAM = true;

Fix this by using the preprocessor macro HAVE_OPENSSL to opt-out the use of
ALLOW_INSECURE_SERVER_DHPARAM in non-OpenSSL builds.

[1] nodejs/node#3890

PR-URL: nodejs/node#4201
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Rod Vagg <rod@vagg.org>

src/node.cc

@@ -3054,7 +3054,9 @@ static void ParseArgs(int* argc,
       SSL3_ENABLE = true;
 #endif
     } else if (strcmp(arg, "--allow-insecure-server-dhparam") == 0) {
+#if HAVE_OPENSSL
       ALLOW_INSECURE_SERVER_DHPARAM = true;
+#endif
     } else if (strcmp(arg, "--help") == 0 || strcmp(arg, "-h") == 0) {
       PrintHelp();
       exit(0);