build:cmake: enable ZSTD legacy support by default

[niamster]

I would like to enable support of legacy formats in CMAKE builds by default.

Backwards compatibility is by default enabled for the VS, make and meson builds, and disabled for cmake and single-file builds.

I think single-file build is special, so it makes sense keep it disabled.

[Cyan4973]

Good point, I agree it makes sense to have cmake build script on par with other build systems (and reference make).

[niamster]

Good point, I agree it makes sense to have cmake build script on par with other build systems (and reference make).

@Cyan4973 Thank you for sharing your opinion! Can you please suggest me what I should do to get an approval?

[niamster]

Thanks @Cyan4973 🙇