Include 3rd parties as SYSTEM about cpm.cmake

[friendlyanon]

I see, so this issue is outside the scope of CPM.cmake and can only be fixed on the dependency's side.

[TheLartians]

I agree that it would be a great to include directories from external dependencies as SYSTEM to avoid warnings that we have no control over. However afaik, I don't think CPM.cmake can do anything here, as the system keyword need to be used inside each library's CMakeLists.txt whenever using target_include_directories on the exported targets.

As this is usually undesirable for library maintainers, this leaves only the option of either ignoring the warnings around the bad #includes:

// save compiler switches
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wpedantic"

// bad headers with problem goes here
#include <library/with/warnings>

// restore compiler switches
#pragma GCC diagnostic pop

Or, of course, you could ask the library maintainer to fix the code that causes the warnings or you fix them in a fork yourself and share the fix in a PR to the main repo.