Build auto_index tool with USE="tools"
Gentoo bug report: https://bugs.gentoo.org/show_bug.cgi?id=529066

--- a/tools/Jamfile.v2.orig	2014-12-24 12:57:32.290880067 +0300
+++ b/tools/Jamfile.v2	2014-12-24 12:58:38.403883250 +0300
@@ -20,6 +20,7 @@
 use-project /boost/regression : regression/build ;
 
 TOOLS =
+    auto_index/build//auto_index
     bcp//bcp
     inspect/build//inspect
     quickbook//quickbook