[Dovecot] building dovecot-sieve against installed dovecot

Hi Timo,

may I request once more to consider the attached patch for dovecot 1.1, it enables the sieve plugin (incl. sievec and sieved, needed for pysieved) to be built against an installed dovecot package, if --enable-header-install additionally installs the liblib.a library in the same directory as dovecot-config.

I have included this patch in the pkgsrc dovecot 1.1 package a while ago.

Geert