boost system dependency isn't available on Ubuntu 26.04 so isn't buildable there

Had same problem in galera MDEV-38978

columnstore/cmake/boost.cmake has the boost(-system) compoent

Ubuntu 26.04 has Boost-1.90.

The boost_system stub was been a library stub since 1.69 and was removed in 1.89.

ref: https://beta.boost.org/users/history/version_1_69_0.html
"Boost.System is now header-only. A stub library is still built for compatibility, but linking to it is no longer necessary. "

Note RHEL7 - which is Boost-1.53.0
RHEL8 might be 1.66.

A galera fix was https://github.com/MariaDB/galera/pull/54 - to make it optional if your confident of the build environment being correct.