a couple of small bugfixes

[Thomas Charles CONWAY]

I've committed these changes.

-- 
Thomas Conway <conway at cs.mu.oz.au>
Nail here [] for new monitor.  )O+


boehm_gc/gc.h:
	ansi-fy a prototype

boehm_gc/linux_threads.c:
	remove a stray 'extern'

scripts/mgnuc.in:
	make the flags for thread_safe compilation achitecture dependent.