[Dovecot] out-of-tree plugins and installed header files

I added --with-headers configure option now to CVS (not enabled by
default). It makes Dovecot install all the useful .h files so that
it's possible to compile out-of-tree plugins without having compiled
Dovecot sources. I'd hope that binary package builders add a new
dovecot-dev package which contains the headers (and $libdir/dovecot- config).

I also updated dovecot-sieve so that it compiles against only the
header files, except then sievec and sieved binaries aren't compiled
(because they require liblib.a).