libiberty: Fix C89-isms in configure tests

libiberty: Fix C89-isms in configure tests

libiberty/

	* acinclude.m4 (ac_cv_func_strncmp_works): Add missing
	int return type and parameter list to the definition of main.
	Include <stdlib.h> and <string.h> for prototypes.
	(ac_cv_c_stack_direction): Add missing
	int return type and parameter list to the definitions of
	main, find_stack_direction.  Include <stdlib.h> for exit
	prototype.
	* configure: Regenerate.