[Configure] disabled plugins are listed by 'dune build @ptests'

configure.in

@@ -607,6 +607,10 @@ if test -f src/plugins/.disabled ; then
   echo "(include_subdirs no)" >> src/plugins/dune
   echo ";; Disabled plugin list:" >> src/plugins/dune
   echo "(data_only_dirs $(cat src/plugins/.disabled))" >> src/plugins/dune
+  echo ";; Test" >> src/plugins/dune
+  echo "(alias (name ptests) (deps (alias ptests_config)))" >> src/plugins/dune
+# "sed -n" is the posix version of "sed --quiet"
+  cat src/plugins/.disabled | sed -n -e 's/^\(.*\)$/(rule (alias "ptests_config") (deps (universe)) (action (echo "Warning: testing with disabled \1 plug-in\n")))/p' >> src/plugins/dune
   chmod a-w src/plugins/dune
   rm src/plugins/.disabled
 fi