Only install dahdi_span_config_hook if DAHDI is enabled

This patch changes the install to only install the hook script if
DAHDI is enabled. It also adds the script to the uninstall task, and
moves the DAHDI_UDEV_HOOK_DIR variable so that it's not between the
_MAKEOPTS variables and their comment.

This allows installs which specify a --prefix to work normally, as
long as they don't enable DAHDI.

Review: https://reviewboard.asterisk.org/r/3972/


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/13@423281 65c4cc65-6c06-0410-ace0-fbb531ad65f3