Since commit 9a6ff950 (Fix for bug where VS2010 did not use .obj files as
part of the build, 2011-04-01) and commit b291d9e7 (VS10: Fix external
objects generated outside target, 2012-03-19) we try to detect whether an
external object file is also a custom command output in the same target.
This is because VS10 includes .obj custom command outputs on the link line
by default.

VS 11 supports a "<LinkObjects ...>false</LinkObjects>" setting in custom
command rules to tell VS not to link the outputs.  From the VS help:

 Specify whether the Inputs and output files with specific extensions (.obj,
 .lib, .res, .rsc) are passed to the linker.

Treat all external object files the same and add "<Object>" settings for them.