Buildbot: Initial work to move linux build environment to CMake

This is so called "seems to work in dry tests" commit which is aimed to switch
linux release environment to CMake.

Some notes:

- There's no special handle of libstdc++, but it wasn't really static for quite
  some time in SCons configuration and nobody really complained.

- It was quite tricky to get OpenMP linked statically with just using some
  configuration so we went ahead and added a special option to CMake now which is
  only exist on Linux and advertised as shouldn't be used.

- Packing is happening manually in slave_pack.py. This is because we have to add
  some really special files to the archive (mesa libraries for example) which we
  can't really handle from CMake/CPack in a nice generic way.

  Don't think it's bad approach, at least crappynness is localized and it's not
  _that_ crappy anyway.

- Windows buildbot should keep working, but needs doublechecing. It's just a
  build folder changed, but you never know what it might imply.

- Some further tweaks are likely needed to ensure all builders are working.

Thanks Campbell for assistance in this patch!