add_subdirectory: Run subdirectory install rules in correct order

Before this change, install rules created by add_subdirectory()
would be executed after all of the top-level install rules, even
if they were declared before the top-level rules. This change
adds a new policy, CMP0082, which interleaves the add_subdirectory()
install rules with the other install rules so they are run in the
correct order.