[RFC] cmake: Platform-dependent checks

[hebasto]

To provide platform-dependent features, CMake suggests two options:

• Platform Variables, and
• CMAKE_SYSTEM_NAME which values depend on CMake's internal platform files, and are not documented anywhere

While the former are concise, they are not enough for all of the supported platforms.

The latter, when using consistently, are pretty verbose (for typing and reading).

The question is to choose or design a concise and consistent way to check target platforms.