Build fix: xdrgen

As discovered in https://review.gluster.org/8762, BSD systems fail to
run xdrgen during glusterfs build. This seems to be caused by a
difference between BSD make and GNU make whith implcit targets. The
former seems to use absolute path here, which means we should not
prepend it with the current directory path, otherwise we have the
directory path twice and the files cannot be found my make.

This is a second attempt after I178123bf6f3d9e963ff5b78839d498f530c05a97
which was broken and reverted in I3c8966288f66d0eafa2e94490e3b64a057b4f2c0

BUG: 1157839
Change-Id: I797c536c319a156b71a42c82cbaf80bbf17b7234
Signed-off-by: Emmanuel Dreyfus <manu@netbsd.org>
Reviewed-on: http://review.gluster.org/9046
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Kaleb KEITHLEY <kkeithle@redhat.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>