CPMAddPackage fails to propagate SYSTEM keyword

[greole]

See title. It seems like if setting the SYSTEM keyword in CPMAddPackage it is not propagated to the cpm_add_subdirectory function. As a result dependencies are build with -I<path> instead of -isystem <path>.

Apparently, it works when specifying SYSTEM YES, which different to the behavior of FetchContent https://cmake.org/cmake/help/latest/module/FetchContent.html

[TheLartians]

Thanks for the issue! This difference to FetchContent has been a conscious decision to avoid (IMO) confusing CMake Syntax where named and positional arguments are weirdly mixed. The CPMAddPackage syntax supports either a single argument or Key/Value pairs, which is why SYSTEM needs a truthy parameter to work. I do understand the confusion though - given the issue this should at least be better documented.