We should also check whether the INTERFACE_ variant of a property
is being read, and in the case of the compile definitions, we should
test the _<CONFIG> suffixed variants. That is already available
through the use of the methods.

This way, we use the ALREADY_SEEN optimization when evaluating
the includes of a target in 'external' generator expressions, ie, those
used in a add_custom_command invokation, as opposed to evaluating the
INCLUDE_DIRECTORIES of a target itself via GetIncludeDirectories.