[Dovecot] LDAP Plugin fails to build in 1.2.rc7

Hi,

The LDAP Plugin fails to build (it doesn't attempt to build it) in 1.2.rc7 because $have_ldap_plugin is checked before it's assigned in configure.in. Patch attached. This was not a problem in 1.2.rc6 and earlier versions.

Cheers, Paul.