Our provider modules are prefixed with gluon- now, and the log message
about a missing provider dir was confusing users.

Ensure packages get rebuilt when gluon.mk changes.

The gluon-web package is basically a stripped-down and refactored version
of the LuCI base.

This call takes an input and an output argument and will copy all files
from the input to the output, while minifying the Lua files.

[Matthias Schiffer: simplified definition, added commit message.]