* Find and add abseil even when BUILD_GEOCODER=OFF
* Treat boolean variables as conditions
* Expose BUILD_SHARED_LIBS as a user configurable option
* Add BUILD_TESTING option
* Wrap test dependencies and targets in if (BUILD_TESTING)
* Refactor library build procedure to respect user defined configurations
* Export cmake package config files so downstream projects can use the library without creating a FindLibPhonenumber
* Bump version number to 8.13.0
Co-authored-by: penmetsaa <penmetsaa@google.com>