Emulation the CMake 3.25 SYSTEM target and directory property

CMake 3.25 added the SYSTEM directory property one can add to add_subdirectory() and FetchContent_Declare and the SYSTEM target property.

I use a library that has a conflicting compilation flag standard leading to a multitude of warnings leaking from the headers. I would like to emulate or reproduce the directory SYSTEM property. Or at least, silence the warnings, exclusively for the headers of this library (i.e., use something like -isystem). I could use #pragma shenanigans but I’m of the opinion that it should be handled by the build system.

The library producing the warnings is imported using FetchContent_Declare.

Ideally the solution would work with CMake 3.20. It should rely on CMake and not user (me) specified compiler flags.

An error occurred: Sorry, new users can only put 2 links in a post. here is the rest of the sources:
FetchContent_Declare: cmakeorg/cmake/help/latest/module/FetchContent.html#command:fetchcontent_declare
add_subdirectory(): cmakeorg/cmake/help/latest/command/add_subdirectory.html#command:add_subdirectory