Mark target_include_directories as SYSTEM when cereal is not standalone

[andrewkcorcoran]

When cereal is included in another CMake project it should mark its include directories as SYSTEM. This prevents compiler errors in cereal if the parent project has more restrictive compiler warnings etc.

See GSL for an example implementation https://github.com/microsoft/GSL/blob/main/CMakeLists.txt#L26 https://github.com/microsoft/GSL/blob/main/include/CMakeLists.txt#L16