- specify that 'all' is a .PHONY target

- add a copyright header to the build_tools Makefile
- remove 'depend' from the 'all' target in agi/ and utils/ since it is handled
  by the main Makefile already


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@35479 65c4cc65-6c06-0410-ace0-fbb531ad65f3