Clipper headers shouldn't be installed

[kintel]

Trying to build & install manifold from source, I noticed that the clipper headers are installed when installing the manifold binaries. Clipper isn't exposed publicly by manifold and the headers should thus not be installed.

Not sure how to fix this atm., just dropping this issue here as I progress through the build&install process.

[pca006132]

Probably just remove this: https://github.com/elalish/manifold/blob/master/manifoldDeps.cmake#L44

[kintel]

Let's wait until I've gone through an entire install cycle. Since clipper shared objects are installed, the installer already leaks that into the system we're installing on. I should probably look into installing clipper separately and see how that flies.