Establish a new distributed CMake architecture with CMake code related to
a source directory moving to be in the subdir in its own CMakeLists.txt.
In particular, there's now one in ./lib which calls through to ones
further down the directory tree like ./lib/plat/xxx, ./lib/roles/xxx etc.
This cuts the main CMakelists.txt from 98KB -> 33KB, about a 66% reduction,
and it's much easier to maintain sub-CMakeLists.txt that are in the same
directory as the sources they manage, and conceal all the details that that
level.
Child CMakelists.txt become responsible for:
- include_directories() definition (this is not supported by CMake
directly, it passes it back up via PARENT_SCOPE vars in helper
macros)
- Addition child CMakeLists.txt inclusion, for example toplevel ->
role -> role subdir
- Source file addition to the build
- Dependent library path resolution... this is now a private thing
in the child CMakeLists.txt, it just passes back any adaptations
to include_directories() and the LIB_LIST without filling the
parent namespace with the details
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