Add SHA256 verification to download scripts

[ddz]

Verify downloaded archives against expected SHA256 checksums in cmake.sha256sum and ninja.sha256sum.

[charlesnicholson]

You seem trustworthy, approved!

[charlesnicholson]

It looks like circle isn't kicking off the build. I have no idea what's going on but found this:

https://support.circleci.com/hc/en-us/articles/360008097173-Why-aren-t-pull-requests-triggering-jobs-on-my-organization-

Would you mind trying to "star" / "watch" this repo and then push a dummy change to see if that kicks off circle? If not, I'll adopt it and apply it under my account but give you credit...

[charlesnicholson]

Ah nevermind, I found the circleci setting to build on forked PR's. It probably needs a nudge from you, though, maybe you could add an empty commit?

Otherwise there was another form, just send me your SSN, DOB, and credit card and that should work.

[charlesnicholson]

This looks better:

[ddz]

I added a small commit to kick off the CI build. All of my PII is en route under separate cover (because security).