configure: use -r option for pr when listing enabled config options.

This fixes ugliness when configure outputs its (empty) results on HPUX
(maybe some other UNIXes too):

Enabled indevs:
pr:  -- empty file
Signed-off-by: Mans Rullgard <mans@mansr.com>

configure

@@ -3143,7 +3143,7 @@ echo
 for type in decoder encoder hwaccel parser demuxer muxer protocol filter bsf indev outdev; do
     echo "Enabled ${type}s:"
     eval list=\$$(toupper $type)_LIST
-    print_enabled '_*' $list | sort | pr -3 -t
+    print_enabled '_*' $list | sort | pr -r -3 -t
     echo
 done