Use configure settings for git-subtree

Include config.make.autogen in the git-subtree contrib area to pick up
settings for prefix and other such things.

Signed-off-by: David A. Greene <greened@obbligato.org>

contrib/subtree/Makefile

@@ -1,3 +1,6 @@
+-include ../../config.mak.autogen
+-include ../../config.mak
+
 prefix ?= /usr/local
 mandir ?= $(prefix)/share/man
 gitdir ?= $(shell git --exec-path)