Respect autotools docdir variable

Change-Id: If20d0806a925b880e0ba9bbc66604cae6c51ce6b
BUG: 859861
Signed-off-by: Kacper Kowalik (Xarthisius) <xarthisius.kk@gmail.com>
Reviewed-on: http://review.gluster.org/3971
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Anand Avati <avati@redhat.com>