Disable Erasure Code xlator if MMX is missing

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

This is a backport of Id9fe87db33e00c5630c1f3633ebd50fc4ebaec4d

BUG: 764655
Change-Id: I880109e42838b5b35ce68508eeb0f8f005faf6f1
Signed-off-by: Emmanuel Dreyfus <manu@netbsd.org>
Reviewed-on: http://review.gluster.org/8381
Reviewed-by: Kaleb KEITHLEY <kkeithle@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Harshavardhana <harsha@harshavardhana.net>