Add .PHONY to Makefile

For make tasks non-file dependent, you should use the .PHONY directive

[http://www.gnu.org/software/make/manual/make.html#Phony-Targets](http://www.gnu.org/software/make/manual/make.html#Phony-Targets)
Merge pull request #1 from cpjolicoeur/patch-1

Add .PHONY to Makefile