Include config.mak in doc/Makefile

config.mak.autogen is already there.  Without this change it is not
possible to override mandir in config.mak.

Signed-off-by: Uwe Kleine-König <ukleinek@informatik.uni-freiburg.de>
Signed-off-by: Junio C Hamano <junkio@cox.net>

Documentation/Makefile

@@ -37,6 +37,7 @@ INSTALL?=install
 DOC_REF = origin/man
 
 -include ../config.mak.autogen
+-include ../config.mak
 
 #
 # Please note that there is a minor bug in asciidoc.