fix(build): separate preprocessor definitions

[kellen-miller]

Description

GYP concatenates adjacent "POSIX," "__STDC_FORMAT_MACROS" literals into one malformed compiler definition. Separate the entries so both macros are defined correctly. Closes #210.

• Code changes have been tested

Summary

- "POSIX," "__STDC_FORMAT_MACROS",
+ "POSIX",
+ "__STDC_FORMAT_MACROS",

Evidence

• Before: Preprocessing with the generated -DPOSIX,__STDC_FORMAT_MACROS emits an invalid macro-name warning and leaves __STDC_FORMAT_MACROS undefined.
  After: Separate -DPOSIX -D__STDC_FORMAT_MACROS defines both macros to 1 without that warning.
• macOS ARM64, Node 24.13: source rebuild (node-pre-gyp rebuild --napi_build_version=3) and npm test pass.
• git diff --check passes. Repository-wide npm run lint stops on existing formatting in unchanged tests/test.js; this PR only changes binding.gyp.

Merge Danger

Door: two-way

Blast Radius: build

No codec source, compiler warning settings, or runtime JavaScript changes.