[libigl] Always install as header-only library

[FabienPean]

In #14376, triplet *-windows-dynamic version was updated to rely on the library as header-only while leaving the *-static triplet intact. However, the static library archive still suffers from the same reported issues: huge size and not all combination of explicitly instantiated templates exist. This PR forces installs of libigl as header-only library for both static and dynamic triplets. (Particularly useful for vcpkg on linux where the default triplet has static linkage by default)

Let me know if this PR is an agreeable change on your side.

[FabienPean]

The PR shows failure for another port.

[JackBoosY]

Maybe we should add an new feature header-only to this port instead of force it to be a header only port.

[FabienPean]

Added header-only as a feature

[BillyONeal]

The "header-only" feature doesn't comply with our guidelines for features, namely, that they do not provide "alternative" selections. See https://github.com/microsoft/vcpkg/blob/master/docs/maintainers/maintainer-guide.md#do-not-use-features-to-control-alternatives-in-published-interfaces

Crucially, any downstream consumer who is passing -ligl (or similar to link the static lib) would be broken by turning on this "feature".

It might be acceptable to make such a chance unconditional if the current users of the port think that makes sense.

/cc @patrikhuber @deGravity @xarthurx

[ras0219-msft]

Since the comments in the upstream project indicate that it is a "header only library", I think the right way forward is to always just do that.

With this change, LGTM!