Fix the Makefile's dependency tracking

Turns out that .depend's mtime changes when you modify files in there
and since every .cpp file depended on .depend, make regenerated everything.
Not good.

Fix this by inlining the mkdir for .depend.


git-svn-id: https://znc.svn.sourceforge.net/svnroot/znc/trunk@1027 726aef4b-f618-498e-8847-2d620e286838