There is an updated pull request by Chocimier against master on the void-packages repository

https://github.com/Chocimier/void-packages-org postgresql12
https://github.com/void-linux/void-packages/pull/18121

[WIP] New package: postgresql12
Still some things to do, like fixing pg_config so postgis can be build for 12, adding postgresql-libs9.6 for rebuilding postgis, maybe behind option.

A patch file from https://github.com/void-linux/void-packages/pull/18121.patch is attached