galera needs packaging script changes to successfully build

https://github.com/MariaDB/galera/pull/15 - ci link

• enables rpm builds to occur on all RPM distros

https://github.com/MariaDB/galera/pull/14

• s390x on Ubuntu 2004 required g++-10, but the build system was overriding the CXX environment variable. This removes the override.

https://github.com/MariaDB/galera/pull/12

• not all RPM builders have scons, and its not needed any more. Easy to remove requirement.

https://github.com/MariaDB/galera/pull/10
dpkg isn't in the buiders, however dpkg-buildpackage which is used is

The success of these can be seen in the dev environment where these have been merged*.

https://buildbot.dev.mariadb.org/#/grid?branch=mariadb-4.x

Note *: excludes first one which may be merged by the time this is looked at - https://github.com/vladbogo/galera/pull/4