Build auto_index tool with USE="tools"
See also: https://bugs.gentoo.org/529066

--- a/tools/Jamfile.v2
+++ b/tools/Jamfile.v2
@@ -18,6 +18,7 @@
     ;
 
 TOOLS =
+    auto_index/build//auto_index
     bcp//bcp
     inspect/build//inspect
     quickbook//quickbook