Now that we're processing a LINK_INTERFACE_LIBRARIES string, it
can contain targets. Make sure they are extracted for
namespacing purposes.

This needs to be restricted to strings which can actually have
targets named in them. For example, this is not done for
INTERFACE_COMPILE_DEFINITIONS, because even if there is a target
named 'foo', the string 'foo' in that property means that '-Dfoo'
will be set when compiling.