Add a compiler feature test for -Wunused-variable.
Merge topic 'tid-system-argument' into next

668ce55 Add a compiler feature test for -Wunused-variable.
Add the INTERFACE_SYSTEM_INCLUDE_DIRECTORIES target property.

Unlike other target properties, this does not have a corresponding
non-INTERFACE variant.

This allows propagation of system attribute on include directories
from link dependents.
Merge topic 'tid-system-argument' into next

6925dc3 Add the INTERFACE_SYSTEM_INCLUDE_DIRECTORIES target property.