you are using slackware current: in the updated glibc in -current some rpc headers are deprecated in favour of libtirpc ones.
if you want to build it, I personally think your best options for now, until things on -current stabilize, are either:
- to reinstall the missing headers: you can find a (unofficial) testing slackbuild I wrote for them here
from which you can make a package.
Install it and dovecot should build fine.
You can uninstall the rpcnis-headers package after building dovecot, but keep it at hand as other software might still need those headers for building.
- to build it against libtirpc
(these are my testing build scripts for this library not yet in slackware), but you will have to modify the official build script a little
(here I also updated it for 2.0.16).
This seems to work too (tested).
I suggest you also to use slackbuilds.org's slackbuild
for building dovecot, a lot of the dirty work is done already for you simply running it, courtesy of Alan Hicks.