Eliminated the possibility of empty <variablelist> elements in enum

references (invalid docbook, apache fop doesn't like it).



[SVN r40282]

tools/boostbook

@@ -1 +1 @@
-Subproject commit e6ce9ce1a543d5ba4c8f1c3b5d234ad725268444
+Subproject commit d2d2c4b90a628240ffd496dab10d9d17fed5d404