configure.ac: migrate from AM_CONFIG_HEADER to AC_CONFIG_HEADERS

AM_CONFIG_HEADER was deprecated.
timeout bugfix from Debian #470038
afuse.c: timeout format friendlier to OSX sscanf

To reproduce:

#include <stdio.h>
#include <stdint.h>
#include <string.h>
int main() {
	uint64_t foo, bar;
	sscanf("180", "%Lu", &foo);
	sscanf("180", "%llu", &bar);
	printf("%li, %li\n", (long int) foo, (long int) bar);
	return(0);
}

===> 140733193388212, 180