www/rssroll: Prepare for base system default PIE

PIE with static libraries is currently unsupported in FreeBSD, which
results in multiple errors when building with PIE. Explicitly disable
PIE for now.

PR: 253275
Submitted by: Dawid Górecki <dgr@semihalf.com>
Approved by: kevans, Nikola Kolev (maintainer)
Differential Revision: https://reviews.freebsd.org/D28863