LU-4052 build: simplify autogen.sh

LU-4052 build: simplify autogen.sh

The concept of conditional subdirectories dates back to the days
when we had Lustre spread across multiple CVS repositories.  For
years now we have used git, and the tree is entirely available
at all times.

Therefore we can simplify autogen.sh a bit more.

Change-Id: I3d7c8e5e93b57c492bb61554a546ceb6d90b2ffe
Signed-off-by: Christopher J. Morrone <morrone2@llnl.gov>
Reviewed-on: http://review.whamcloud.com/7840
Tested-by: Hudson
Reviewed-by: Dmitry Eremin <dmitry.eremin@intel.com>
Reviewed-by: Minh Diep <minh.diep@intel.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>