The first commit is only cosmetic and can be dropped (but I like to work on modern/valid shell syntax).
The second commit remove hardcoded path. I have rpm in non standard location and it this macro made all my build fail (I was mad when I searched where it was called but forgot the ~/.rpmmacros: I now understand this warning from redhat https://access.redhat.com/documentation/fr-fr/red_hat_enterprise_linux/8/html/packaging_and_distributing_software/advanced-topics#using-the-custom-macros_more-on-macros)
The third commit is to reduce content of ~/.rpmmacros in hope we can get rid of it one day.

Next step is copy __arch_install_post upstream ?