CDRIVER-4637 Remove ENABLE_SRV=AUTO, clean up SRV URI support (#1301)

Remove AUTO for ENABLE_SRV, refactor .pc generation

This AUTO wasn't doing anything, and behaved very similar to plain ON.
Now we define an interface library that provides the resolving library.
Some cleanup to the ResSearch lookup module.

* Use ENABLE_SRV to toggle SRV support rather than detection macros
* Add distinct feature flag for SRV toggle.
* Refactor pkg-config generation to use target properties
  This change uses target properties to fill in the slots in the
  generated pkg-config files. Previously this was unstructured strings
  and flag juggling.
* Tweak method of detecting name resolution
Co-authored-by: Roberto C. Sánchez <roberto@connexer.com>