app_dir statement is deprecated.
As far as I can tell no one is using it.
If you do use it, it will only affect initial generation with -c. Then it specifies the name of the build directory. Normally the name of the build directory is taken from the app name as h2xs does it.
Note that only a single path element is allowed.
You cannot specify depth as in 'under/here'.
For that you need the equally deprecated
To see an example, build:
bigtop -c example.bigtop all
Note that the build directory is called
useless_dir instead of