r25697: make *clean doesn't work after a svn up when some *.mk files were removed

r25697: make *clean doesn't work after a svn up when some *.mk files were removed

the error is:
SAMBA_4_0/source> make clean
make: *** No rule to make target `lib/ldb/samba/config.mk', needed by `Makefile'.  Stop.

the problem is:
Makefile: config.status $(MK_FILES)
	./config.status

so now we let the MK_FILES variable empty for the *clean targets
if gnu make is detected, we should later test if this construct
is portable to other make implementations and remove the check for gnu make.

metze
(This used to be commit 4c8e539af1e8f14bb449c92db3376dec12344e5b)