add the 'clean', 'clean-depend', and 'dist-clean' targets as .PHONY targets

add the 'clean', 'clean-depend', and 'dist-clean' targets as .PHONY targets
since they are targets that do not have resulting files and are never listed
as prerequisites to real targets.  Using .PHONY in this manner improves make
performance by never having to check for resulting files.

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@35415 65c4cc65-6c06-0410-ace0-fbb531ad65f3