This seems related to http://www.jetbrains.net/jira/browse/IDEADEV-8990 which is supposed to be fixed in 5755 which i'm currently using.
But I still get an error when i do the following
create a new single module project.
Create a new (empty) file named build.xml wherever in the project
Adding this file as an ant build file (using right click menu), in order to take advantage of completion, fails with a "cannot add build files from excluded directories" even if the file is in the src dir