Use the sourcemeta_enable_simd() in Blaze

[jviotti]

I'm making SIMD flags opt-in to make it easier to release cross-platform
binaries for tools like the JSON Schema CLI (as some target machines
will not support SIMD).

While we don't use SIMD directly, auto-vectorisation does, and saw
issues in the wild with the JSON Schema CLI binaries not working on
certain machines.

Signed-off-by: Juan Cruz Viotti jv@jviotti.com

[michaelmior]

@jviotti This looks fine to me. Any reason not to merge?

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