Add BackgroundTask.cxx/h to the VC7.1 build system.
Add BackgroundTask.cxx/h to the VC7.1 build system.

kongr45gpen/bzflag-import-1,kongr45gpen/bzflag-import-1,kongr45gpen/bzflag-import-1,kongr45gpen/bzflag-import-1,kongr45gpen/bzflag-import-1,kongr45gpen/bzflag-import-1