configure: Check sasl.h existence only when using --with-ldap. Patch by Pascal Volk.
configure: Check sasl.h existence only when using --with-ldap.
Patch by Pascal Volk.

--HG--
branch : HEAD

jkerihuel/dovecot,jkerihuel/dovecot,jwm/dovecot-notmuch,jkerihuel/dovecot,jwm/dovecot-notmuch,jwm/dovecot-notmuch,jkerihuel/dovecot,jwm/dovecot-notmuch,jkerihuel/dovecot,jwm/dovecot-notmuch