CMake paradigm shift in FC source

[luzpaz]

JFYI, There is a proposal for revamping CMake:
FreeCAD/FreeCAD#2477

relevant:
https://github.com/qingfengxia/FreeCAD_Mod_Dev_Guide/blob/master/chapters/2.FreeCAD_source_code_structure.md

https://github.com/qingfengxia/FreeCAD_Mod_Dev_Guide/blob/master/chapters/A2.cmake_cheatsheet.md