Merge topic '17870-effective-system-name'

4597396784 Apple: Use include_guard() within compiler config
e95b3fd9bb Apple: Use CMAKE_EFFECTIVE_SYSTEM_NAME to share compiler info
84f9f63fcc Modules: Introduce CMAKE_EFFECTIVE_SYSTEM_NAME to lookup compiler info
9fa0f2eb56 CMakeFindPackageMode: Perform platform-specific initialization

Acked-by: Kitware Robot <kwrobot@kitware.com>
Merge-request: !1964