vpx: avoid confusing meson configure output when checking for vpx versions

vpx: avoid confusing meson configure output when checking for vpx versions

Used to print:
|Run-time dependency vpx found: YES 1.7.0
|Message: libvpx provides VP8 encoder interface (vpx_codec_vp8_cx_algo)
|Message: libvpx provides VP8 decoder interface (vpx_codec_vp8_dx_algo)
|Message: libvpx provides VP9 encoder interface (vpx_codec_vp9_cx_algo)
|Message: libvpx provides VP9 decoder interface (vpx_codec_vp9_dx_algo)
|Dependency vpx found: YES (cached)
|Dependency vpx found: NO found '1.7.0' but need: '>=1.8.0'
|Run-time dependency vpx found: NO (tried pkgconfig and cmake)

We can check the version of the found dep in a way that
doesn't produce this confusing output.