Disable Erasure Code xlator if MMX is missing

Erasure Code xlator requires MMX instruction set.
Disable building it if MMX is missing.

BUG: 764655
Change-Id: Id9fe87db33e00c5630c1f3633ebd50fc4ebaec4d
Signed-off-by: Emmanuel Dreyfus <manu@netbsd.org>
Reviewed-on: http://review.gluster.org/8366
Reviewed-by: Xavier Hernandez <xhernandez@datalab.es>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>