Fix SIGSEGV in mark_from called from do_local_mark if WRAP_MARK_SOME

Fix SIGSEGV in mark_from called from do_local_mark if WRAP_MARK_SOME

Issue #179 (bdwgc).

Parallel marker is disabled (for now) if there is a chance of
scanning unmapped data roots (because GC_do_local_mark is not
prepared to deal with memory protection faults gracefully).

* mark.c [MSWIN32 || USE_PROC_FOR_LIBRARIES] (WRAP_MARK_SOME): Move
definition to gcconfig.h.
* include/private/gcconfig.h [WRAP_MARK_SOME && PARALLEL_MARK]
(PARALLEL_MARK): Undefine (as not implemented yet).