install,export: Maybe transform OBJECT libraries to INTERFACE libraries

Brad Kingrequested to merge
brad.king/cmake:object-library-export-as-interface into master

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.

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