For now do not allow an OBJECT library to reference other object
libraries. Teach cmTarget::ComputeLinkImplementation to include the
languages of object libraries used by a target.
This library type can compile sources to object files but does not link
or archive them. It will be useful to reference from executable and
normal library targets for direct inclusion of object files in them.
Diagnose and reject the following as errors:
* An OBJECT library may not be referenced in target_link_libraries.
* An OBJECT library may contain only compiling sources and supporting
headers and custom commands. Other source types that are not normally
ignored are not allowed.
* An OBJECT library may not have PRE_BUILD, PRE_LINK, or POST_BUILD
commands.
* An OBJECT library may not be installed, exported, or imported.
Some of these cases may be supported in the future but are not for now.
Teach the VS generator that OBJECT_LIBRARY targets are "linkable" just
like STATIC_LIBRARY targets for the LinkLibraryDependencies behavior.
Some per-target information and logic is common to all generators.
Some of that information is currently stored in cmTarget but that
should be reserved for the configure step. Create a class to hold
per-target information for generators. On construction classify
sources from the target and store them in separate members. This
classification is already implemented separately in each generator.
Brad King