[mpeg2-dev] Update to config.{guess,sub} needed

[Eric L. <i...@ha...>]

Hello, I'm building libmpeg2 on riscv64, and the configure script is 
too old to know such a new architecture.

Running `autoreconf -fi` fixes the problem, but an update to those 
scripts should be a better solution (if someone with write access is 
active, which probably won't be).

Thank you anyway for developing this project!