fix(pkg): install the ob-standalone-setup links under the install-time prefix

[guimard]

Reported by @orontee on #272: the "Build Debian Package" job logs two CMake Errors and still passes.

CMake Error: failed to create symbolic link '…/_CPack_Packages/Linux/DEB/open-bastion-0.6.2-Linux/usr/local/sbin/ob-standalone-setup': No such file or directory
CMake Error: failed to create symbolic link '…/usr/local/share/man/man8/ob-standalone-setup.8': No such file or directory

It is not specific to #272 — main has the same two lines.

Cause

The two links were made by install(CODE) against CMAKE_INSTALL_FULL_SBINDIR / CMAKE_INSTALL_FULL_MANDIR. Those are baked at configure time (/usr/local/...), but cpack installs under /usr. The parent directory does not exist in the staging tree, create_symlink fails, execute_process ignores the result, and cpack exits 0 — with a .deb that has no ob-standalone-setup and no ob-standalone-setup.8.

dpkg-buildpackage (dh, prefix /usr) and rpmbuild (%cmake) configure with /usr, so the paths matched there: the released packages were not affected (and both list the link explicitly, so their builds would fail without it).

Fix

• The links are made in the build tree at configure time and installed with install(FILES … DESTINATION ${CMAKE_INSTALL_SBINDIR}) (resp. ${CMAKE_INSTALL_MANDIR}/man8). A relative destination follows the install-time prefix and DESTDIR. Works with our cmake_minimum_required(3.10).
• The CI package job now fails when cpack reports a CMake Error, since cpack itself does not.

Verified locally

$ cpack -G DEB    # before: 2 × CMake Error, link absent from the .deb
$ cpack -G DEB    # after: 0 × CMake Error
$ dpkg-deb -c open-bastion-0.6.2-Linux.deb | grep standalone
lrwxrwxrwx root/root 0 … ./usr/sbin/ob-standalone-setup -> ob-bastion-setup
lrwxrwxrwx root/root 0 … ./usr/share/man/man8/ob-standalone-setup.8 -> ob-bastion-setup.8

and cmake -DCMAKE_INSTALL_PREFIX=/usr + DESTDIR=… cmake --install (the dh/rpm path) installs both as symlinks.

https://claude.ai/code/session_01Bh9aBGroMwsqpv4q5foHT3