This helps make the DSO stuff more portable;

* "no-dso" option available in Configure so that all DSO methods will
  return NULL, overriding any support the platform might otherwise
  have built.
* dlfcn_no_h config string now available rather than just dlfcn. This
  is for platforms that have dlfcn.h functions but do not have (or
  need) the dlfcn.h header file.