message: Adding cesium-native build instructions using UE4 provided Linux CLang toolchain
diff: @@ -48,8 +48,25 @@ Cesium for Unreal depends on Cesium's high-precision geospatial C++ library - [C
  1. Clone the repository using `git clone --recursive git@github.com:CesiumGS/cesium-unreal.git`.
  2. From the `cesium-unreal/extern` directory, run the following commands to build `cesium-native`.
  + * CMake configuration on Windows platform:
  +
  ```cmd
  cmake -B build -S . -G "Visual Studio 15 2017 Win64" # Optionally use "Visual Studio 16 2019"
  + ```
  +
  + * CMake configuration on Linux platform:
  +
  + ```cmd
  + export UNREAL_ENGINE_DIR=<path_to_unreal_engine>
  + export UNREAL_ENGINE_COMPILER_DIR=$UNREAL_ENGINE_DIR/Engine/Extras/ThirdPartyNotUE/SDKs/HostLinux/Linux_x64/v17_clang-10.0.1-centos7/x86_64-unknown-linux-gnu/bin/
  + export UNREAL_ENGINE_LIBCXX_DIR=$UNREAL_ENGINE_DIR/Engine/Source/ThirdParty/Linux/LibCxx
  +
  + cmake -B build -S . -DCMAKE_CXX_COMPILER=$UNREAL_ENGINE_COMPILER_DIR/clang++ -DCMAKE_CXX_LINK_EXECUTABLE=$UNREAL_ENGINE_COMPILER_DIR/ld.lld -DCMAKE_CXX_FLAGS="-nostdinc++ -I$UNREAL_ENGINE_LIBCXX_DIR/include -I$UNREAL_ENGINE_LIBCXX_DIR/include/c++/v1" -DCMAKE_POSITION_INDEPENDENT_CODE=ON
  + ```
  +
  + * CMake build (any platform):
  +
  + ```
  cmake --build build --config Debug --target install # Optional, recommended for debugging
  cmake --build build --config Release --target install # Can optionally compile with --config RelWithDebInfo or MinSizeRel.
  ```
  