build: disable `TCP_NODELAY` for emscripten

build: disable `TCP_NODELAY` for emscripten

In WebAssembly, using `TCP_NODELAY` fails with:
```
* Could not set TCP_NODELAY: Protocol not available
```

Add a new feature macro in `curl_setup.h` telling whether `TCP_NODELAY`
is known to be supported at runtime, when defined at compile-time.

Keep `TCP_NODELAY` guards at their current positions to ensure the
necessary headers (e.g. `netinet/tcp.h` and `netinet/in.h`) define it.

Reported-by: Jeroen Ooms
Fixes #17974
Closes #18155