Set import library for Clipper2 target when using system version

[imciner2]

On Windows, the import library is also needed to link against the Clipper2 DLL, and that must be specified using a different target property.

We noticed this when building manifold in Julia on Windows: JuliaPackaging/Yggdrasil#8712 (comment)

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.27%. Comparing base (d437097) to head (8fdf5b5).
Report is 25 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #815      +/-   ##
==========================================
- Coverage   91.84%   89.27%   -2.57%     
==========================================
  Files          37       61      +24     
  Lines        4976     8439    +3463     
  Branches        0      927     +927     
==========================================
+ Hits         4570     7534    +2964     
- Misses        406      905     +499     

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[elalish]

Thank you!