Investigate ways to make cesium-native easy to consume from other CMake projects

[kring]

i.e., with a find_package or external_project_add or whatever is considered best practice these days.

[lilleyse]

@kring can this be closed?

[kring]

Sure, it's not a very useful issue.

The problem it's describing still stands, though. It's not as easy as it should be to add cesium-native as a dependency to another project.