Avoid Wundef trigger

[starseeker]

When the Manifold headers are included in BRL-CAD, one of our compile flags doesn't like MANIFOLD_EXCEPTIONS being undefined - if I'm not mistaken, we can just check for the definition here, without worrying about its specific value.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.04%. Comparing base (d437097) to head (511e3b3).
Report is 58 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #861      +/-   ##
==========================================
- Coverage   91.84%   90.04%   -1.80%     
==========================================
  Files          37       65      +28     
  Lines        4976     9069    +4093     
  Branches        0      946     +946     
==========================================
+ Hits         4570     8166    +3596     
- Misses        406      903     +497     

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[elalish]

Indeed, that looks like a typo to me.