wmain: build as c++ instead of c

Allows to keep exiv2 c++ only.

Signed-off-by: Rosen Penev <rosenp@gmail.com>

app/wmain.cpp

@@ -0,0 +1,40 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+#include <windows.h>
+
+extern int __cdecl main(int, char*[]);
+
+int wmain(int argc, wchar_t* argv[]) {
+  char** args;
+  int nbytes = static_cast<int>(sizeof(char*) * (argc + 1));
+  HANDLE heap = GetProcessHeap();
+
+  for (int i = 0; i < argc; ++i)
+    nbytes += WideCharToMultiByte(CP_UTF8, 0, argv[i], -1, nullptr, 0, nullptr, nullptr);
+
+  args = reinterpret_cast<char**>(HeapAlloc(heap, 0, nbytes));
+  args[0] = reinterpret_cast<char*>(args + argc + 1);
+
+  for (int i = 0; i < argc; ++i)
+    args[i + 1] = args[i] + WideCharToMultiByte(CP_UTF8, 0, argv[i], -1, args[i], nbytes, nullptr, nullptr);
+
+  args[argc] = nullptr;
+
+  argc = main(argc, args);
+  HeapFree(heap, 0, args);
+  return argc;
+}
+
+int WINAPI wWinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, LPWSTR lpCmdLine, int nCmdShow) {
+  (void)hInstance;
+  (void)hPrevInstance;
+  (void)lpCmdLine;
+  (void)nCmdShow;
+
+  int argc;
+  wchar_t** argv;
+  argv = CommandLineToArgvW(GetCommandLineW(), &argc);
+  argc = wmain(argc, argv);
+  LocalFree(argv);
+  return argc;
+}