Support installing a target multiple times
This is required in order to support installing targets for both an *nix-style SDK and a macOS framework setup.
The following list at least needs to be implemented:
- a policy for the new behavior;
moving install-related properties off of targets and on to install commands:
INSTALL_INTERFACE(this will not be simple :/ );
- support for exporting a target multiple times.
An alternative would be to, as a post-install step, take a *nix-style SDK install tree and create macOS bundles from them, fixing IDs and such in the process. The exported
targets files would need massaging as well.