Fix warnings in public headers

[kring]

Somewhere along the way, we started identifying our public header files to CMake as SYSTEM. This is the right-ish thing to do for libraries that consume cesium-native via submodule / add_directory, so that a project stricter warnings won't flag a bunch of warnings in cesium-native headers. But it means that legitimate warnings in the header files, even when building cesium-native, are ignored. I noticed this because apparently some old versions of CMake don't treat SYSTEM headers specially, and the build had a whole bunch of warnings.

So we should:

• Tweak our CMake config so that the headers are SYSTEM outside cesium-native, but non-system within it.
• Fix the warnings