build: cleanup: reformat use of AC_ARG_ENABLE

1. Use AS_HELP_STRING to format help.

2. Use autoconf variable for statement.

Change-Id: Iaf32239a946e299711a2f0de660f3ac8ec1fed59
Signed-off-by: Alon Bar-Lev <alonbl@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/18928
Tested-by: Dan Kenigsberg <danken@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>