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>)

Help/command/install.rst

@@ -183,6 +183,11 @@ export called ``<export-name>``.  It must appear before any ``RUNTIME``,
 ``LIBRARY``, ``ARCHIVE``, or ``OBJECTS`` options.  To actually install the
 export file itself, call ``install(EXPORT)``, documented below.
 
+:ref:`Interface Libraries` may be listed among the targets to install.
+They install no artifacts but will be included in an associated ``EXPORT``.
+If :ref:`Object Libraries` are listed but given no destination for their
+object files, they will be exported as :ref:`Interface Libraries`.
+
 Installing a target with the :prop_tgt:`EXCLUDE_FROM_ALL` target property
 set to ``TRUE`` has undefined behavior.