Simplify Leaf and its sub-classes.

Change-Id: I711db26bedee2bebdd39ffd8b637fa8918c16ff9
diff --git a/wscript b/wscript
index ee2684a..0f0ea36 100644
--- a/wscript
+++ b/wscript
@@ -50,6 +50,7 @@
         source =  bld.path.ant_glob(['src/**/*.cpp', 'src/**/*.proto']),
         use = 'BOOST NDN_CXX',
         includes = ['src', '.'],
+        export_includes=['src', '.'],
         )
 
     # Unit tests