qsort_r: Fix macrology for platforms that lack the function. * m4/stdlib_h.m4 (gl_STDLIB_H): Check for qsort_r. (gl_STDLIB_H_DEFAULTS): Initialize HAVE_QSORT_R. * modules/stdlib (Makefile.am): Substitute HAVE_QSORT_R. * lib/stdlib.in.h (qsort_r): Provide declaration if the function does not exist. * m4/qsort_r.m4 (gl_FUNC_QSORT_R): Use AC_CHECK_FUNCS to test whether the function exists. * modules/qsort_r: Add comments.