Replace WIN32 with _MSC_VER

At some point this guard started failing and I don't know when that
happened.

client/connection.cpp

@@ -111,7 +111,7 @@ Connection::Connection() noexcept
  uint16_t port = Socket::DefaultPort;
  std::string serverName = DEFAULT_REMOTE_HOST;
 
-#if defined(WIN32)
+#if defined(_MSC_VER)
  WSADATA wsaData;
  if (WSAStartup(MAKEWORD(2, 2), &wsaData) != 0) {
  std::cerr << "Windows sockets failed to initialise." << std::endl;
@@ -192,7 +192,7 @@ Connection::~Connection()
  }
  }
 
-#if defined(WIN32)
+#if defined(_MSC_VER)
  WSACleanup();
 #endif
 }

common/socket.cpp

@@ -20,7 +20,7 @@
 #include <iostream>
 #include "hints.h"
 
-#if defined(WIN32)
+#if defined(_MSC_VER)
  #include <WinSock2.h>
  #include <WS2tcpip.h>
 #else
@@ -71,11 +71,11 @@ RemoteCL::Socket::Socket()
  throw Error();
  }
 #if !defined(REMOTECL_DISABLE_KEEP_ALIVE)
-#if defined(WIN32)
+#if defined(_MSC_VER)
  using ValTy = const char*;
 #else // everyone else
  using ValTy = const void*;
-#endif // WIN32
+#endif // _MSC_VER
  int val = -1;
  int result = setsockopt(mSocket, SOL_SOCKET, SO_KEEPALIVE, reinterpret_cast<ValTy>(&val), sizeof(val));
  if (result != 0) {
@@ -181,7 +181,7 @@ Socket::PeerName RemoteCL::Socket::getPeerName() noexcept
 void RemoteCL::Socket::close() noexcept
 {
  assert(mSocket != InvalidSocket);
-#if !defined(WIN32)
+#if !defined(_MSC_VER)
 #define closesocket(X) ::close(X)
 #endif
  closesocket(mSocket);

common/socket.h

@@ -21,7 +21,7 @@
 #include <cstdint>
 #include <utility> // for std::swap
 
-#if defined(WIN32)
+#if defined(_MSC_VER)
 #include <WinSock2.h>
 #endif
 
@@ -31,7 +31,7 @@ namespace RemoteCL
 class Socket
 {
 public:
-#if !defined(WIN32)
+#if !defined(_MSC_VER)
  using SocketTy = int;
  static constexpr SocketTy InvalidSocket = -1;
 #else