Instead of using sed to strip away the source dir this now uses make's notdir.
$(notdir this/is/an/example) results in example.
This also moved the generated dependency files from .depend/extra into .depend/
Since we need the extra/ path prefix for the modules from modules/extra, that
one is added back via addprefix.
git-svn-id: https://znc.svn.sourceforge.net/svnroot/znc/trunk@1610 726aef4b-f618-498e-8847-2d620e286838