should FetchContent_MakeAvailable make the package's INTERFACE_INCLUDE_DIRECTORIES a system directory?
I assume the essentially universal use of
FetchContent is to download a third party package. In such a case, it serves the role of a "system" header: compiler warnings aren't useful.
It seems like in this case the
INTERFACE_INCLUDE_DIRECTORIES specified by the package should be marked as a system include directory.
If this is not the correct default, perhaps the
SYSTEM keyword could be included to
FetchContent_Declare to specify that this is desired.