Apparently QNX declares dl_iterate_phdr and friends in sys/link.h
rather than link.h.  This patch updates libbacktrace to check there.
This fixes https://github.com/ianlancetaylor/libbacktrace/issues/86.
Bootstrapped and ran libbacktrace testsuite on x86_64-pc-linux-gnu.
Committed to mainline.

Ian

* 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.