LU-709 build: remove old BGL support option

Remove the old and obsolete --enable-bgl configure option.
Lustre no longer supports the old kernel that BG/L systems
use, and we do not use this opton on the later BG/P and
BG/Q systems.

Change-Id: Iea0b893fd67fe5242fa65ebc8993c1f2b0863ae6
Signed-off-by: Christopher J. Morrone <morrone2@llnl.gov>
Reviewed-on: http://review.whamcloud.com/4841


Tested-by: Hudson
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Yang Sheng <yang.sheng@intel.com>