Fix -Wundef issues in src/inet. (#29221)

* __MBED__ should be tested for via ifdef.
* HAVE_SO_BINDTODEVICE should be an actual config macro.

src/inet/InetConfig.h

@@ -272,4 +272,15 @@
 #endif
 #endif // INET_CONFIG_UDP_SOCKET_PKTINFO
 
+/**
+ *  @def HAVE_SO_BINDTODEVICE
+ *
+ *  @brief
+ *    Should be set to 1 if the SO_BINDTODEVICE option to setsockopt is
+ *    available.
+ */
+#ifndef HAVE_SO_BINDTODEVICE
+#define HAVE_SO_BINDTODEVICE 0
+#endif
+
 // clang-format on

src/inet/InetInterface.cpp

@@ -586,7 +586,7 @@ short InterfaceIterator::GetFlags()
             mIntfFlags       = intfData.ifr_flags;
             mIntfFlagsCached = true;
         }
-#if __MBED__
+#ifdef __MBED__
         CloseIOCTLSocket();
 #endif
     }
@@ -670,7 +670,7 @@ uint8_t InterfaceAddressIterator::GetPrefixLength()
     {
         if (mCurAddr->ifa_addr->sa_family == AF_INET6)
         {
-#if !__MBED__
+#ifndef __MBED__
             struct sockaddr_in6 & netmask = *reinterpret_cast<struct sockaddr_in6 *>(mCurAddr->ifa_netmask);
             return NetmaskToPrefixLength(netmask.sin6_addr.s6_addr, 16);
 #else  // __MBED__

src/inet/TCPEndPointImplSockets.cpp

@@ -797,19 +797,19 @@ void TCPEndPointImplSockets::HandlePendingIO(System::SocketEvents events)
         // The socket being writable indicates the connection has completed (successfully or otherwise).
         if (events.Has(System::SocketEventFlags::kWrite))
         {
-#if !__MBED__
+#ifndef __MBED__
             // Get the connection result from the socket.
             int osConRes;
             socklen_t optLen = sizeof(osConRes);
             if (getsockopt(mSocket, SOL_SOCKET, SO_ERROR, &osConRes, &optLen) != 0)
             {
                 osConRes = errno;
             }
-#else
-            // On Mbed OS, connect blocks and never returns EINPROGRESS
-            // The socket option SO_ERROR is not available.
+#else  // __MBED__
+       // On Mbed OS, connect blocks and never returns EINPROGRESS
+       // The socket option SO_ERROR is not available.
             int osConRes     = 0;
-#endif
+#endif // !__MBED__
             CHIP_ERROR conRes = CHIP_ERROR_POSIX(osConRes);
 
             // Process the connection result.