Add rtti support to SKIA

[omartijn]

This is necessary on UNIX platforms when using the skia library in an application built with rtti itself.

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[dg0yt]

Is this really a feature or a triplet setting?

[omartijn]

Is this really a feature or a triplet setting?

I think having this as a triplet setting is quite a nice idea, and I support that as an idea.

However, we already have other ports that have this as a feature, e.g. llvm, glslang, bullet3, mesa, halide`. For consistency, I'd argue we merge it like this for now.

If we turn this into a triplet-feature we also need to ensure it's properly supported for all ports, which I think is quite a bit of work, so I think that deserves it's own PR.

[dg0yt]

At least I'm surprised to see EXTRA_CXX_CFLAGS getting quick "approval", despite the usual "follow CMake toolchain settings". Add did you check that there isn't some explicit gn config stuff? Does the flag depend on actual compiler?

[dg0yt]

Is this specific to clang, https://github.com/llvm/llvm-project/issues/66117?

[ADKaster]

fwiw, the official docs https://skia.org/docs/user/build/#quickstart have an example doing

bin/gn gen out/RTTI     --args='extra_cflags_cc=["-frtti"]'

Which doesn't seem like a particularly sane way to do this.

There is a gn config in skia/gn/portable/BUILD.gn for add_rtti that is applied to the dng_sdk vendored dependency. but there doesn't seem to be a way to enable it globally for all targets in the upstream gn build.

[dg0yt]

@ADKaster The question is whether a port feature should add compiler flags.

Triplet and cmake toolchain control the compiler flags. Even per-port settings are possible. The skia port already passes cmake toolchain settings into gn.