Fix symbolize detection on macOS

[take-cheeze]

Since by detecting as UNIX first will end up as Linux: https://cmake.org/cmake/help/latest/variable/UNIX.html

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 63.73%. Comparing base (2075ae8) to head (77665b0).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1138 +/- ## ========================================== - Coverage 64.03% 63.73% -0.30% ========================================== Files 20 20 Lines 2580 2606 +26 Branches 888 903 +15 ========================================== + Hits 1652 1661 +9 - Misses 663 677 +14 - Partials 265 268 +3 ``` [see 9 files with indirect coverage changes](https://app.codecov.io/gh/google/glog/pull/1138/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=google)

[take-cheeze]

The warning message isn't printed in PR CI: https://github.com/google/glog/actions/runs/11684741346/job/32543647005?pr=1138#step:7:283 Master branch CI: https://github.com/google/glog/actions/runs/11361540607/job/31601487654#step:7:236

[sergiud]

Thanks!