Make `hack/gen-content.sh` discover GNU `grep` and `sed` on macOS

[embik]

Running hack/gen-content.sh falls apart when running on macOS if grep doesn't point to GNU grep due to the -P flag heavily being used. This is normally not the case (you can install it via homebrew and override the grep name, but that's not the default), so I've adjusted the script to discover $GREP and $SED (just in case sed also acts up and I didn't notice) and set them to the GNU versions (which are prefixed with a g on installation). The discovery heuristics are simple but seem to be effective.

I'm not sure if this is something that is interesting to this repository (since you can also run this in a container), just wanted to submit it in case it would be useful to the community. Feel free to tell me that this will not be merged.

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[stmcginnis]

I've had to do something similar to this in the past. I don't have a macOS system to test on right now, but this looks like it covers the detection and use throughout.

At any rate, testing with the changes on a Linux system doesn't show anything broken by the change, so: /lgtm

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[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: embik, mrbobbytables

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