Workaround a problem with GCC 4.9 on x86_64 systems.

configure.ac:
	Apply the same workaround for a problem with GCC labels
	on x86_64 systems with GCC 4.9 as we do with GCC 4.8.
	(XXX we should probably disable -ftree-dominator-opts on all
	GCCs that support it, but I'm not sure how widespread the problem
	with it is -- I'll look into that separately.)

runtime/mercury_stack_trace.c:
	Avoid a warning from GCC.