Export stop/start_world_external only for multi-threaded builds

Export stop/start_world_external only for multi-threaded builds
(fix of commit bb91f03)

Issue #173 (bdwgc).

Also, update GC_world_stopped value in GC_stop_world_external and
GC_start_world_external.

* include/gc.h (GC_stop_world_external, GC_start_world_external): Do not
declare unless GC_THREADS; add comment.
* misc.c (GC_stop_world_external, GC_start_world_external): Do not
define unless THREADS; add assertion that GC is initialized.
* misc.c [THREADS && THREAD_LOCAL_ALLOC] (GC_stop_world_external): Set
GC_world_stopped to true after STOP_WORLD; add assertion that the
world is not stopped.
* misc.c [THREADS && THREAD_LOCAL_ALLOC] (GC_start_world_external): Set
GC_world_stopped to false before START_WORLD; add assertion that the
world is stopped.
* tests/test.c [GC_PTHREADS && CPPCHECK] (main): Add UNTESTED for
GC_stop_world_external and GC_start_world_external.