New comment by ericonr on void-packages repository

https://github.com/void-linux/void-packages/pull/30032#issuecomment-894574407

Comment:
My `_GNU_SOURCE` suggestion was apparently a bad idea for one of the packages, feel free to move it back to a conditional for musl.

Turns out glibc will define its own `fdiv` function if you include `<tgmath.h>` with `_GNU_SOURCE` defined, so we can't define it for glibc to avoid polluting the namespace and causing conflicts.