As spotted in #7656, macOS installer files built on Big Sur fail signature
verification on macOS 10.11. This is because Big Sur productsign no longer
supplies the SHA-1 hash, and 10.11 does not know how to read the SHA-256
hash.
Replace the productsign flow with a flow based on
http://users.wfu.edu/cottrell/productsign/productsign_linux.html . This
uses the xar tool to digitally sign the installer packages, with both
SHA-1 and SHA-256 hashes.
The xar tool is somewhat tricky to build, so is checked in (as binary!)
compiled for Mac.
To build a Mac package, run make_pkg.sh (which invokes the signing flow)
followed by mac_notarize.sh which adds the notarization.
