Fix macro usage

What's Changed

• Conditional fix for supporting glibc 3.0 by @sdavtaker in #537
• use macros consistently by @DmitriyMusatkin in #538
• Fix macro usage by @DmitriyMusatkin in #539
• stop checking errno after failed aws_fopen() by @graebm in #540

New Contributors

• @sdavtaker made their first contribution in #537

Full Changelog: v0.13.12...v0.13.13