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

lib/cf-socket.c

@@ -109,7 +109,7 @@ static void set_ipv6_v6only(curl_socket_t sockfd, int on)
 
 static void tcpnodelay(struct Curl_easy *data, curl_socket_t sockfd)
 {
-#ifdef TCP_NODELAY
+#if defined(TCP_NODELAY) && defined(CURL_TCP_NODELAY_SUPPORTED)
   curl_socklen_t onoff = (curl_socklen_t) 1;
   int level = IPPROTO_TCP;
   char buffer[STRERROR_LEN];

lib/curl_setup.h

@@ -1125,6 +1125,11 @@ int getpwuid_r(uid_t uid, struct passwd *pwd, char *buf,
 #define USE_HTTP3
 #endif
 
+/* WebAssembly builds have TCP_NODELAY, but runtime support is missing. */
+#ifndef __EMSCRIPTEN__
+#define CURL_TCP_NODELAY_SUPPORTED
+#endif
+
 /* Certain Windows implementations are not aligned with what curl expects,
    so always use the local one on this platform. E.g. the mingw-w64
    implementation can return wrong results for non-ASCII inputs. */