Build instructions: update configure to use specific gcc versions

gcc/g++ must be at least version 12 that means that on core (building LO) we need to use that see https://wiki.documentfoundation.org/Development/BuildingOnLinux#openSUSE and on online the same goes.
* Add `CC="ccache gcc-12" CXX="ccache g++-12"` flags  to the `./configure` line under building online for each distro if needed
* Test it

