[libigl] Fix dynamic build and enable header-only mode

[FabienPean]

This is an attempt at getting rid of libigl huge static .lib file and getting the header-library mode. I could make it work on my machine (Windows VS2019 Clang 10) with the current set of modifications. Not necessarily optimal nor "clean", but hopefully a good start to resolve the issue.

Let me know if anything is missing or feel free to take over from here.

• What does your PR fix? Fixes [libigl] Enable header-only mode #14371

• Which triplets are supported/not supported? Have you updated the CI baseline?
  Dynamic library version works on windows but not the static, and other platform untested. I don't know what is the CI baseline.

• Does your PR follow the maintainer guide?
  I think so.

[FabienPean]

Alright so, despite compiling successfully, the baseline x64-windows-static is not actually usable. It leads to /failifmismatch: mismatch detected for '_ITERATOR_DEBUG_LEVEL'. Current changes also successfully pass the CI and leads to same problem.

However my question is, should vcpkg_check_linkage(ONLY_DYNAMIC_LIBRARY) be imposed and ci.baseline.txt updated to libigl:x64-windows-static=fail or stick to "if it compiles it is fine"

Otherwise, I am out of idea to make the static build actually be usable. Since it does not alter the original behavior (on x64-windows) the PR should be ok, but could be simplified if dynamic only is decided (I most probably added too many if(LIBIGL_USE_STATIC_LIBRARY) initially hoping to solve the static build issue).

Let me know your opinion on the subject

[FabienPean]

The CI x86_windows instance is hogging.

For convenience, I made the gist with the test files here (requires building igl from head for fixes on writeDAE): https://gist.github.com/FabienPean/9c1a6c5eea6b304d0e1241024e0387b1

[LilyWangL]

/azp run

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[strega-nil]

Thanks @FabienPean :)