use the AX_TLS macro to find correct thread-local storage class compiler directive

git-svn-id: http://arthurdejong.org/svn/nss-pam-ldapd/nss-pam-ldapd@1921 ef36b2f9-881f-0410-afb5-c4e39611909c
use pthreads thread-local storage as fallback mechanism if compiler doesn't provide a keyword for TLS

git-svn-id: http://arthurdejong.org/svn/nss-pam-ldapd/nss-pam-ldapd@1922 ef36b2f9-881f-0410-afb5-c4e39611909c