I didn't see the sysconfdir issue locally, I've tried explicitly passing sysconfdir to Cmake. If that does not help, please debug this a bit more.
I replaced your fix with
CMAKE_ARGS+= CMAKE_INSTALL_FULL_SYSCONFDIR=${PKG_SYSCONFDIR}
(i.e. added "_FULL") because I saw that that variable was used for
constructing several paths. Unfortunately, this didn't fix anything.
During install, some files are still installed under the wrong prefix:
-- Installing: /opt/pkgsrc/pkg-hrz/pkgsrc/net/tigervnc/work.pkgsrc-sol11i86/.destdir/opt/pkg-hrz/20201118/man/man1/vncpasswd.1 -- Installing: /opt/pkgsrc/pkg-hrz/pkgsrc/net/tigervnc/work.pkgsrc-sol11i86/.destdir/opt/pkg-hrz/20201118/sbin/vncsession -- Installing: /opt/pkgsrc/pkg-hrz/pkgsrc/net/tigervnc/work.pkgsrc-sol11i86/.destdir/etc/opt/pkg-hrz/20201118/pam.d/tigervnc -- Installing: /opt/pkgsrc/pkg-hrz/pkgsrc/net/tigervnc/work.pkgsrc-sol11i86/.destdir/opt/pkg-hrz/20201118/man/man8/vncsession.8 -- Installing: /opt/pkgsrc/pkg-hrz/pkgsrc/net/tigervnc/work.pkgsrc-sol11i86/.destdir/opt/pkg-hrz/20201118/libexec/vncserver
I'll keep looking... -- Jörn Clausen BITS - Bielefelder IT-Servicezentrum https://www.uni-bielefeld.de/bits