Allow Clipper2 to be supplied externally when using Manifold as a submodule

[kintel]

• Checks Clipper2_FOUND before attempting to find Clipper2
• Add imported Clipper2 library only when trying pkg_config

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.54%. Comparing base (926d2e7) to head (567637f).
Report is 4 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1115      +/-   ##
==========================================
+ Coverage   91.46%   91.54%   +0.07%     
==========================================
  Files          30       30              
  Lines        5908     5904       -4     
==========================================
+ Hits         5404     5405       +1     
+ Misses        504      499       -5     

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[elalish]

What are you waiting for to make this not a Draft?

[kintel]

What are you waiting for to make this not a Draft?

My own manual testing across platforms, which takes some time. I mostly wanted to make sure this is conceptually ok with you, plus make sure Manifold's tests pass.

[kintel]

This is good to go now!

[pca006132]

LGTM. I'm wondering if we should do something similar for other dependencies, e.g. tbb.

[kintel]

Yeah, it could make sense to align the deps lookups. I didn't run into the same issue with tbb as we're always supplying it as a system dependency from the OpenSCAD side.

[kintel]

Another idea would be to hide all the details in custom Find macros, but I didn't want to touch too much as it's so time consuming to test it..

[pca006132]

We typically just write some simple test in our CI, like https://github.com/elalish/manifold/blob/master/scripts/test-cmake-subdir.sh

It should not be too hard to test. I can have a look at it later, and probably deal with #1105. Our CI jobs will probably grow by 5... I'm starting to think if we are kind of abusing GitHub action :P

[elalish]

Thanks!