Fix regression in qsort_r BSD detection

Fix regression in qsort_r BSD detection

We can use AC_CHECK_FUNCS to detect if qsort_r is available on
the system or not since it will unconditionnally define
HAVE_QSORT_R, which we don't want since on BSD, qsort_r isn't usable
for us, so we don't want to have HAVE_QSORT_R defined on such platforms.
By using AC_CHECK_FUNC instead, we can defer defining HAVE_QSORT_R until
we have actually tested it's usable.

https://bugzilla.gnome.org/show_bug.cgi?id=651920