Libbacktrace

QNX uses sys/link.h rather than link.h for dl_iterate_phdr

Fixes #86

	* configure.ac: Check for sys/link.h.  Use either link.h or
	sys/link.h when checking for dl_iterate_phdr.
	* elf.c: Include sys/link.h if available.
	* configure, config.h.in: Regenerate.