Better macOS support

[alvarogalloc]

I'm using Halley with clang 17 in a macOS Big Sur box. With little changes in Cmake files, the build succeeds. Here are some of the problems:

• cannot build halley-metal due to lack of update.
• halley-opengl works but the editor just goes black after 1s of opening it, though it works just fine for games as for now.
• I have to provide my deps through vcpkg or building them myself because halley-deps has only windows binaries.
• some error messages in halley-cmd when trying to convert hlsl shaders.

These are some problems I've seen so far, if I can I might make the changes myself and make a pr.