libmpeg2 0.5.1 was released in mid-2008, let's bump the requirement
and get rid of version-dependent code paths. There's still
avdec_mpeg2video for those who are stuck on ancient distros which
we don't target any more.
Also fixes build with MSVC, which doesn't like #if #else #endif
inside macro arguments (like the GST_DEBUG_OBJECT in line 941).