At least on my setup (x86_64-linux) the main loop is never entered.

After some inspection, I believe this is because WindowShouldClose() returns a bool, which has a size of 1 byte on my system, but the temporary variable holding the result of the call is 8 bytes (I think), and so it may contain garbage in the upper 7 bytes that will mess with the while condition.

    ...

    InitWindow(800, 600, "Hello, from B");
    SetTargetFPS(60);
    while (!WindowShouldClose()) { // <-- here
        if (IsKeyPressed(32)) {
            paused = !paused;
        }

    ...

After discovering that your compiler supports bits operations, I tested this hypothesis with this change:

     InitWindow(800, 600, "Hello, from B");
     SetTargetFPS(60);
-    while (!WindowShouldClose()) {
+    while (!(WindowShouldClose() & 1)) {
         if (IsKeyPressed(32)) {
             paused = !paused;
         }

And it worked as intended.

I feel that making extrn function declarations aware of the full function signature (with args and return types) is not aligned with the spirit of the project, and so manually handling non word sized external values seems the way to go. I can make a PR if necessary.

Very cool project by the way. The compiler's hack / test / debug cycle is top notch. And the btest output is crazy good.