[account42]

For Windows, MinGW has an independent implementation of the complete build environment so you don't need any VS tools, import libraries or headers. For macOS there is no such complete build root but the actual tools are available - e.g. clang is always a cross-compiler able of targetting all supported architectures and for the linker Apple's own ld64 is open source and we also have lld now although I am not certain the macOS target status.

> AFAIK you can have clang-cl on Linux

clang-cl is a frontent for MSVC command-line compatibility - you don't even need that for cross-compiling, just -target and then whatever -fms-* extensions you need for your non-portable code. But yes, Clang doesn't come with its own SDK so you will need headers and import libraries and possibly other tooling (although the LLVM procect covers a lot).