share: don't use SSL unless enabled

Don't even declare the struct members for disabled features

Introducing the CURLSHE_NOT_BUILT_IN return code for the share interface
when trying to set a sharing option that has been disabled (or not
enabled) in the library.

lib/strerror.c

@@ -384,6 +384,9 @@ curl_share_strerror(CURLSHcode error)
   case CURLSHE_NOMEM:
     return "Out of memory";
 
+  case CURLSHE_NOT_BUILT_IN:
+    return "Feature not enabled in this library";
+
   case CURLSHE_LAST:
     break;
   }