Set symbol visibility to hidden by default on macOS and Linux

[kduske]

Describe the pull request

• What does your PR fix?

Fixes linker warnings when linking against libraries with inconsistent visibility settings (main project has symbols hidden, linked library has symbols visible).

• Which triplets are supported/not supported? Have you updated the CI baseline?

  osx, linux, no

• Does your PR follow the maintainer guide?

  Maybe -- I'll double check.

• If you have added/updated a port: Have you run ./vcpkg x-add-version --all and committed the result?

  I am still working on this PR

[kduske]

@Neumann-A There are a few failures that are due to libraries not exporting symbols. I'm not sure that the list is exhaustive because I don't know what the CI tests entail. Is there a test for each library where a program links against it?

Assuming that the list is complete, we could fix these libraries by changing their ports to set symbol visibility to default explicitly -- essentially the opposite of what I did for poly2tri here.

I did see some warnings about CMP0063 not being set even though I set it in the toolchain files. I'd have to investigate why this happens.

There is one particularly nasty caveat however, and it has to do with different compilers merge typeinfo objects for multiple instances of identical types. I don't fully understand this and I'm probably using the wrong terms, but this stackoverflow question and the answers explain it quite well: https://stackoverflow.com/questions/19496643/using-clang-fvisibility-hidden-and-typeinfo-and-type-erasure

I have seen this problem in action and it can be very subtle and hard to diagnose. I worry that a lot of vcpkg users might run into this and will have to spend a lot of time diagnosing it if we just turn it on for every library. What do you think?

[Cheney-W]

Is work still being done for this PR?

[kduske]

@Cheney-W Not from my side. I'm not sure if @Neumann-A wants to follow up on this, though.

[Neumann-A]

Just close. Was worth a try ;)