Issue #6580 - Unify directory definitions to one section of Makefile (casper)

- Maybe we should define proper directories for OS/X too?
- Should OpenBSD and NetBSD follow the FreeBSD rules?


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@18130 65c4cc65-6c06-0410-ace0-fbb531ad65f3