This will make the `next` buildsystem no longer the `next`, but the
`actual`. The idea is to simplify the build, removing scripts, to
make the platform buildsystem generated by CMake more compatible
with IDEs and make development more streamlined. It will also make
cross compilation easier.
Run it in parallel with the old build system. This will offer
a migration path and make sure we keep everything working at
the cost of running many CI bots. New stuff is prefixed with "next".
Appveyor already got migrated to the next buildsystem.
After merging, we will follow-up removing duplicated core bots.
