src: add HAVE_OPENSSL directive to openssl_config

Currently when building with the following configuration options:
$ ./configure --without-ssl && make

The following link error is reported:

Undefined symbols for architecture x86_64:
  "node::openssl_config", referenced from:
      node::Init(int*, char const**, int*, char const***) in node.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see
invocation)

Adding an HAVE_OPENSSL directive around this code allows the build to
pass.

PR-URL: nodejs#11618
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>

src/node.cc

@@ -4233,8 +4233,10 @@ void Init(int* argc,
   if (config_warning_file.empty())
     SafeGetenv("NODE_REDIRECT_WARNINGS", &config_warning_file);
 
+#if HAVE_OPENSSL
   if (openssl_config.empty())
     SafeGetenv("OPENSSL_CONF", &openssl_config);
+#endif
 
   // Parse a few arguments which are specific to Node.
   int v8_argc;