New comment by tornaria on void-packages repository

https://github.com/void-linux/void-packages/pull/30032#issuecomment-894916605

Comment:
The "fix" for the `_GNU_SOURCE` issue was bad, which caused the musl build to fail check.

Based on this and the comments in https://github.com/wbhart/flint2/pull/988, I removed all patches and reverted back to using `CFLAGS=-D_GNU_SOURCE` in the template, with the care that for `arb` it has to be conditional to avoid a conflict.

Now all tests should pass... :crossed_fingers: