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.

BUG: 1129939
Change-Id: I8f469c5741a85b6e8d8f6299a9540b3d64611d2f
Signed-off-by: Emmanuel Dreyfus <manu@netbsd.org>
Reviewed-on: http://review.gluster.org/8786
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Jeff Darcy <jdarcy@redhat.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>