[snip]
I’m not running Postgres server. It is running on the different machines. All I need is to build the client library. Why can’t I? Why it should matter what software (OS/compiler) I’m using? As long as I can successfully build my software and test it it should be fine.
David made it pretty clear: the PGDG changed the toolchain, and decided not to support an OS that went EOL three years ago, and a compiler that's only getting security updates: "There is no guarantee or real attempt to keep modern supported versions running on software obsoleted before they even came into existence."