The version number is useful to check the libavutil version against which
the library was compiled at run-time, which in turn may be useful to deal
with binary compatibility issues.
Refactoring mmx2/mmxext YASM code with cpuflags will force renames.
So switching to a consistent naming scheme beforehand is sensible.
The name "mmxext" is more official and widespread and also the name
of the CPU flag, as reported e.g. by the Linux kernel.