Fix apkg rpm build

[qupfer]

On Almalinux9 (and probably other RHEL9 like systems) the "apkg build" packaging step for RPMs seems broken.

I'm not sure, what realy happen but some suggestions.

The cmake file install/copies a file with the line
install(FILES "${CMAKE_SOURCE_DIR}/pam/netopeer2.conf" DESTINATION "${CMAKE_INSTALL_SYSCONFDIR}/pam.d")

CMAKE_INSTALL_SYSCONFDIR is set to "etc" by default, what looks to be correct.

However, maybe related to cmake GNUInstallDirs and and a set/default CMAKE_INSTALL_PREFIX it's expanded internal to /usr/etc, resulting the file mentioned above is put to .../usr/etc/pam.d/

The RPM Spec file on the other side, expect it in %{_sysconfdir} aka /etc and fails.
A solution is to set CMAKE_INSTALL_SYSCONFDIR to the absolut path /etc.

Thats what this MR did. :-)

[michalvasko]

Thanks, since I cannot think of a better fix, merging.