buildtest: ignore empty folders

`compile_test.py` fails if there are empty folders in `/examples` or
`tests`. This is not a problem for Travis, because it always has a clean
repo.

For the average users there is a problem if they work on multiple
branches and one branch has new examples or tests. If they don't delete
the residual `bin` folders, then `compile_test.py` will print out these
applications as having failed.

This change checks for the existence of a `Makefile` in the application
folder. Also I added a `(n/N)` indicator, because I like it. :)