Declare empty target as phony otherwise they are caught by the '%' rule

Declare empty target as phony otherwise they are caught by the '%' rule

Without that build on the build farm is broken:

'build' finished successfully (8.162s)
WAF_MAKE=1 python ./buildtools/bin/waf --targets=everything
    Selected embedded Heimdal build
    Waf: Entering directory `/memdisk/build/samba_4_0_test/bin'
	Selected embedded Heimdal build
	Waf: Leaving directory `/memdisk/build/samba_4_0_test/bin'
	target 'everything' does not exist
	make: *** [everything] Error 1

Change-Id: Id5bc37f803efd764f00c37f8add560d6de2e3a2f
Signed-off-by: Matthieu Patou <mat@matws.net>
Reviewed-by: Michael Adam <obnox@samba.org>