Teach the `install` and `export` commands to support installing and
exporting `OBJECT` libraries without their object files. Transform
them to `INTERFACE` libraries in such cases.
For `install(TARGETS)`, activate this when no destination for the object
files is specified. For `export`, activate this only under Xcode with
multiple architectures when we have no well-defined object file
locations to give to clients.
Teach install() and export() to handle the actual object files.
Disallow this on Xcode with multiple architectures because it
still cannot be cleanly supported there.
Co-Author: Brad King <brad.king@kitware.com>
