Include config.mak.autogen in the doc Makefile

... to install documentation relative to the path set with configure's
--prefix option.

Signed-off-by: Jonas Fonseca <fonseca@diku.dk>
Signed-off-by: Junio C Hamano <junkio@cox.net>

Documentation/Makefile

@@ -33,6 +33,8 @@ man7dir=$(mandir)/man7
 
 INSTALL?=install
 
+-include ../config.mak.autogen
+
 #
 # Please note that there is a minor bug in asciidoc.
 # The version after 6.0.3 _will_ include the patch found here: