postproc: update SIMD variant selection

the cpu SIMD selection code removed here dates from a time when vlc had
options for disabling use of select SIMD variants, from before postproc
added cpu auto-detection ([1]), and from before postproc seems to have
added SSE2 ([2] and [3]).

we are purging MMX/MMXEXT from vlc v4.0-dev, and thus have an interest in
removing the corresponding MMX/MMXEXT bits here. rather than just removing
those lines, alongside adding an entry for SSE2 though, i have instead
chosen to convert the code to use auto-detection, which avoids having to
keep the block of code explicitly enabling implementations in sync with the
set of implementations available.

note, the version of postproc in contribs is very old, pre-dating the
SSE2 and CPU feature auto-detection enhancements. accordingly i have had to
ensure that we define `PP_CPU_CAPS_AUTO` ourselves when not found, as had
been done for `PP_CPU_CAPS_ALTIVEC`. effectively, for users like myself on
linux with a new enough version, the auto-detection will work correctly and
now make use of SSE2, which we were ignoring before; while where the
contrib package is used, its use will fall back to the C implementation
until such time that the contrib gets updated.

[1]: https://github.com/FFmpeg/FFmpeg/commit/59d686f100863d00b8f171dd891e893c2bfd951e
[2]: https://github.com/FFmpeg/FFmpeg/commit/4e264d1c79cfae8c3e05aacf77e350ed1b6d7e4b
[3]: https://github.com/FFmpeg/FFmpeg/commit/f48cddfe4cf04e2d6e802d12e973301ff5a1a9a8