fix(gatsby): add runtime check for react 17

[wardpeet]

Description

Make sure we only make it a default for react 17+. Not all libraries have moved to support automatic mode.

Related Issues

Related #26652

[wardpeet]

Tested with latest react.

I've added to a page

/** @jsx createElement  */
import React, { createElement } from "react"

with previous gatsby I got

gatsby-blog-starter\src\pages\index.js: pragma and pragmaFrag cannot be set when runtime is automatic.

now I don't get an error 👍

[pieh]

Also for some paper trail - this become a problem now because React@16.14 added the jsx transform ( https://github.com/facebook/react/blob/master/CHANGELOG.md#16140-october-14-2020 ) and was released yesterday and Mdx in particular doesn't seem compatible for now with it (?), causing errors like Ward described above ( https://app.circleci.com/pipelines/github/gatsbyjs/gatsby/50975/workflows/341312ae-79b4-41c2-a6d9-c31e5cae080d/jobs/526766 )

[jquense]

ya'll this is annoying. We can't upgrade to react 17 because the ecosystem doesn't support this automatic runtime yet

[wardpeet]

@jquense what's the issue? We add the automatic rule when you're using react 17

[jquense]

you still have the same errors as folks this was trying to fix. e.g. emotion, mdx, system-ui and most libraries that depend on jsx pragma do not support the automatic runtime and break even on react 17.

Basically this isn't just a matter of using react 17. Libraries have to all update to support the new jsx transform. opting everyone into a poorly supported feature isn't great

[jquense]

basically by opting everyone into this for react 17 it gates everyones ability to upgrade to react 17 to whenever all these other libraries (which do support react 17) figure out jsx automatic runtime support

[wardpeet]

@jquense okay so we should disable it completely. Sounds good.

[wroughtec]

or make it a configuration so the user is able to enable to disable depending upon their needs