drop -Wcase-qual when using --enable-warnings because it was causing too much noise (r1908 aka. 2a73fa1 from 0.9)

git-svn-id: http://arthurdejong.org/svn/nss-pam-ldapd/nss-pam-ldapd-0.8@1942 ef36b2f9-881f-0410-afb5-c4e39611909c