Even if we don't agree on a userspace ABI, this is still fine-ish, as long as you can statically link everything you need. Unfortunately the nerds maintaining the core libraries REALLY don't want you to do that, and the answer to "how do I build a portable Linux GUI program" goes more or less like:

"Build musl libc statically, set up a toolchain to use it, build libc++ for that toolchain, get libwayland, link that statically (which their build scripts don't support, roll your own), get xcb,libxau,libxwhatever and build those statically as well, and implement TWO platform layers, dynamically checking for wayland support. There's like 5 different ways to set your window icon. Yes, you need to implement all of them. Now for loading the graphics API......."

On Windows it's a call to RegisterClassW followed by CreateWindowW.