Can you try starting Dovecot by hand with 'LD_PRELOAD=libpthread.so dovecot" and see if this fixes things please?
this also 'fixes' the problem with courier-authlib-mysql on a amd64/current box (19/10/2014) without LD_PRELOAD: /usr/local/libexec/courier-authlib/authdaemond INFO: modules="authsqlite authmysql", daemons=5 INFO: Installing libauthsqlite INFO: Installation complete: authsqlite INFO: Installing libauthmysql INFO: file not found with LD_PRELOAD: LD_PRELOAD=/usr/local/lib/libauthmysql.so.0.0 /usr/local/libexec/courier-authlib/authdaemond INFO: modules="authsqlite authmysql", daemons=5 INFO: Installing libauthsqlite INFO: Installation complete: authsqlite INFO: Installing libauthmysql INFO: Installation complete: authmysql -luis
