[libvpx] Change default target on Unix

[kafeg]

Set default libvpx target to the 'x86_64-darwin17-gcc'. It's macOS 10.13 (High Sierra)

It's required to support the latest CPU features like hardware acceleration.

[JackBoosY]

Please update the version info. See documentation.

[JackBoosY]

Waiting for merge #11496.

[kafeg]

Done

[JackBoosY]

LGTM, thanks for this PR!

[strega-nil]

You should probably also do:

    set(LIBVPX_TARGET "--target=...")

and then instead of passing --target=${LIBVPX_TARGET}, pass ${LIBVPX_TARGET} so that on, say, FreeBSD, it still builds.

[kafeg]

You should probably also do:

    set(LIBVPX_TARGET "--target=...")

and then instead of passing --target=${LIBVPX_TARGET}, pass ${LIBVPX_TARGET} so that on, say, FreeBSD, it still builds.

Hey! I did it by another way, by set set(LIBVPX_TARGET "generic-gnu") # use default target

[strega-nil]

Alright, cool, I'm happy with that. Thanks @kafeg :)