Improvements to CMake build

[kring]

• Reference cesium-native submodule by relative path, so it works with https or SSH connections to github.
• Remove glm submodule; it's already in cesium-native.
• Include cesium-native with just add_directory and reference its libs and headers with just target_link_libraries.
• Remove all the ExternalProject and manual header/lib configuration stuff.
• Use a new version of cesium-native that fixes a bunch of build things (https://github.com/CesiumGS/cesium-native/pull/10)

[baothientran]

Thanks @kring, the new cmake setup works for me and no glm dependency is needed