Call sites such as those in the VS global generator that are used only
to reject per-config sources will now allow per-config object library
objects.  The corresponding generators have already been taught to deal
with per-config object library files.  Remaining call sites do not need
object library files anyway.

This will later allow `$<TARGET_OBJECTS:...>` generator expressions to
evaluate to values that vary by configuration (e.g. because each
configuration has its own object files).