added comment -- I always have to run twice to build docs because the doxyfile gets updated and causes scons to get confused.  I do not know how to fix this, but I simply run 'scons doc/doxyfile' first.
diff --git a/doc/sconscript b/doc/sconscript
index dc29320..3e4205c 100644
--- a/doc/sconscript
+++ b/doc/sconscript
@@ -20,3 +20,7 @@
                                  TARGZ_BASEDIR = doc_topdir )
         env.Depends( zip_doc_cmd, alias_doc_cmd )
         env.Alias( 'doc-dist', zip_doc_cmd )
+
+    # When doxyfile gets updated, I get errors on the first pass.
+    # I have to run scons twice.  Something is wrong with the dependencies
+    # here, but I avoid it by running "scons doc/doxyfile" first.