Byproducts: collapse full paths of custom target byproducts

Collapse full paths of add_custom_target command byproducts for symmetry with
byproducts passed to the add_custom_command command.

Source/cmAddCustomTargetCommand.cxx

@@ -127,7 +127,7 @@ bool cmAddCustomTargetCommand(std::vector<std::string> const& args,
           }
           filename += copy;
           cmSystemTools::ConvertToUnixSlashes(filename);
-          byproducts.push_back(filename);
+          byproducts.push_back(cmSystemTools::CollapseFullPath(filename));
         } break;
         case doing_depends: {
           std::string dep = copy;