install,export: Maybe transform OBJECT libraries to INTERFACE libraries

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.

Help/command/export.rst

@@ -40,6 +40,13 @@ policy CMP0022 is NEW.  If a library target is included in the export
 but a target to which it links is not included the behavior is
 unspecified.
 
+.. note::
+
+  :ref:`Object Libraries` under :generator:`Xcode` have special handling if
+  multiple architectures are listed in :variable:`CMAKE_OSX_ARCHITECTURES`.
+  In this case they will be exported as :ref:`Interface Libraries` with
+  no object files available to clients.
+
 ::
 
   export(PACKAGE <name>)