documentation: make version fix

The Makefiles in the build directories use the internal make variable
MAKEFILE_LIST which is available from make 3.80 only.  (The patch would be
valid back to 2.6.25)

Signed-off-by: Adam Lackorzynski <adam@os.inf.tu-dresden.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>