Add OBJECT_LIBRARY target type

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.

1 files changed, 2 insertions, 1 deletions

diff --git a/Source/cmTarget.h b/Source/cmTarget.h
index ff05cd3..e2502f0 100644
--- a/Source/cmTarget.h
+++ b/Source/cmTarget.h
@@ -59,7 +59,8 @@ class cmTarget
 public:
   cmTarget();
   enum TargetType { EXECUTABLE, STATIC_LIBRARY,
-                    SHARED_LIBRARY, MODULE_LIBRARY, UTILITY, GLOBAL_TARGET,
+                    SHARED_LIBRARY, MODULE_LIBRARY,
+                    OBJECT_LIBRARY, UTILITY, GLOBAL_TARGET,
                     UNKNOWN_LIBRARY};
   static const char* GetTargetTypeName(TargetType targetType);
   enum CustomCommandType { PRE_BUILD, PRE_LINK, POST_BUILD };