"valid result" for REQUIRED_VARS in find_package_handle_standard_args is too unspecific
tells the vars need to have a "valid result", but this is too unspecific, i.e. set but empty variables are invalid. A reference to https://cmake.org/cmake/help/latest/command/if.html should be added.
This is especially problematic, as pkgconfig by default omits system directories (unless --keep-system-cflags is specified), and find_package_handle_standard_args(FOO FOO_INCLUDE_DIRS) bails out.