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 ✅

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

Additional details and impacted files

@@            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

[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!