Do not hardcode setfattr(1) path

Turn setfattr(1) absolute path into an OS-dependant macro. Let compiler
option override it to fit custom installation if needed.

Backport of I8f469c5741a85b6e8d8f6299a9540b3d64611d2f

BUG: 1138897
Change-Id: I279752f2ec5db1abc25830cb9a23290cc401d517
Signed-off-by: Emmanuel Dreyfus <manu@netbsd.org>
Reviewed-on: http://review.gluster.org/8828
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>