mail/mailest: fix build with base system PIE default

PIE with static libraries is currently unsupported in the FreeBSD base
system. Explicitly disable PIE for in affected ports for now.

PR: 253275 [exp-run]
Submitted by: Dawid Gorecki <dgr@semihalf.com>
Approved by: lwhsu
Approved by: Maintainer timeout
Differential Revision: https://reviews.freebsd.org/D28866