[Bug #18997] Don't define ruby_qsort when POSIX qsort_r is available

	The current code would define ruby_qsort as a wrapper of qsort_s
	when it is available. When both qsort_s and POSIX (GNU) qsort_r
	are available, we should call qsort_r directly instead, and
	the qsort_s wrapper is redundant.
	---
	 util.c | 6 ++++--
	 1 file changed, 4 insertions(+), 2 deletions(-)

	Delete a macro that has never been used, probably added by mistake

	---
	 util.c | 7 -------
	 1 file changed, 7 deletions(-)

	Include headers for `ruby_qsort` only if needed

	If GNU `qsort_r` is available, we use the function and these headers
	are not used.
	---
	 util.c | 4 ++--
	 1 file changed, 2 insertions(+), 2 deletions(-)