ENH: Tolerate repeated link library types

The "debug", "optimized", and "general" link library type specifier
arguments to the target_link_library commands are sometimes repeated in
user code due to variable expansion and other complications.  Instead of
silently accepting the duplicates and trying to link to a bogus library
like "optimized.lib", warn and ignore the earlier specifiers.

Source/cmTargetLinkLibrariesCommand.h

@@ -79,6 +79,8 @@ public:
   
   cmTypeMacro(cmTargetLinkLibrariesCommand, cmCommand);
 private:
+  void LinkLibraryTypeSpecifierWarning(int left, int right);
+  static const char* LinkLibraryTypeNames[3];
 };