This script builds dovecot with support for SSL. As such, you must have
openssl installed - openssl-solibs is not enough in itself to meet this
requirement.  Additionally, dovecot will be compiled with POP3 support.

You should create a dovecot user to run the login processes on your server. 
Something like this should work:
[root@host] # groupadd -g 202 dovecot
[root@host] # useradd -d /dev/null -s /bin/false -u 202 -g 202 dovecot

From the dovecot-new release posting comes this word of advice:
  Remember to read http://wiki2.dovecot.org/Upgrading/2.0 before
  upgrading from v1.x. Although there is automatic config conversion,
  it can't handle everything, so reserve some extra time for upgrading.

See the documentation in /usr/doc/dovecot-*/ to get started.