Makefiles: Use $(PHONY) instead of .PHONY.

Makefiles: Use $(PHONY) instead of .PHONY.

Avoids a warning from Automake in Smake:

src/ui/gui/automake.mk:463: warning: .PHONY was already defined in condition TRUE, which includes condition HAVE_GUI ...
Makefile.am:91:   'src/automake.mk' included from here
src/automake.mk:55:   'src/ui/automake.mk' included from here
src/ui/automake.mk:21:   'src/ui/gui/automake.mk' included from here
po/automake.mk:128: ... '.PHONY' previously defined here
Makefile.am:87:   'po/automake.mk' included from here