Allow caching have_qsort_r which re-enables cross-compiling

https://bugzilla.gnome.org/show_bug.cgi?id=646309

configure.ac

@@ -574,7 +574,7 @@ AC_FUNC_ALLOCA
 AC_CHECK_FUNCS(mmap posix_memalign memalign valloc fsync pipe2)
 AC_CHECK_FUNCS(atexit on_exit timegm gmtime_r)
 # BSD has a qsort_r with wrong argument order
-AC_MSG_CHECKING([for qsort_r])
+AC_CACHE_CHECK([for qsort_r], glib_cv_have_qsort_r, [
 AC_RUN_IFELSE([[
 #define _GNU_SOURCE
 #include <stdlib.h>
@@ -605,9 +605,9 @@ main (int argc, char **argv)
     return 0;
   else
     return 1;
-}]],[have_qsort_r=yes],[have_qsort_r=no])
+}]],[glib_cv_have_qsort_r=yes],[glib_cv_have_qsort_r=no])])
 
-if test $have_qsort_r = yes ; then
+if test $glib_cv_have_qsort_r = yes ; then
   AC_MSG_RESULT([yes])
     AC_DEFINE(HAVE_QSORT_R, 1, [Define to 1 if you have the 'qsort_r' function])
 else

docs/reference/glib/cross.sgml

@@ -109,6 +109,15 @@ chmod a-w win32.cache   # prevent configure from changing it
             built-in version in that case.
 	</para>
       </formalpara>
+      <formalpara>
+         <title>glib_cv_have_qsort_r=[yes/no]</title>
+
+         <para>
+           Whether you have <function>qsort_r()</function> that matches
+           BSD. Defaults to "no", which is safe, since GLib uses a
+           built-in version in that case.
+         </para>
+      </formalpara>
       <formalpara>
          <title>glib_cv_va_val_copy=[yes/no]</title>