config.h unconditionally defines _GNU_SOURCE

[gdt]

Visibility defines are generally problematic, and should be used only when necessary. It's not clear why this is present, and on what systems are needed. This ticket is about minimizing, perhaps reducing, this usage, and adding comments that justify it.

On a NetBSD system, removing it results in a clean build with no warnings. But that's likely not where it's intended.

(This could also lead to identifying and removing non-POSIX usage.)