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.

BUG: 1157839
Change-Id: I178123bf6f3d9e963ff5b78839d498f530c05a97
Signed-off-by: Emmanuel Dreyfus <manu@netbsd.org>
Reviewed-on: http://review.gluster.org/9016
Reviewed-by: Harshavardhana <harsha@harshavardhana.net>
Tested-by: Harshavardhana <harsha@harshavardhana.net>
Reviewed-by: Humble Devassy Chirammal <humble.devassy@gmail.com>