LU-1981 build: External OSD support in configure

LU-1981 build: External OSD support in configure

With configure option "--with-osd=<dir>" one can get additional
(external to main tree) directory to be built with main tree.

Change-Id: Ibe1c1d136d9c8bf8245344791f0f51933266018a
Signed-off-by: Alex Zhuravlev <bzzz@whamcloud.com>
Signed-off-by: Li Wei <liwei@whamcloud.com>
Reviewed-on: http://review.whamcloud.com/4099
Tested-by: Hudson
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>