[MSCOREE] Show a user popup if .NET is missing (#8374)

Otherwise, you wouldn't see anything at all when opening a .NET app, and
the only hint that a .NET runtime is needed would be to see the message
in the debug log, provided a debugger is connected to ReactOS ...

NOTE: `shell32!ShellMessageBoxA()` is used instead of
`user32!MessageBoxA`, because it turns out mscoree isn't importing
from user32 but from shell32 instead (as in Wine's), and I don't want
to modify the list of imported modules.

NOTE 2: This warning function has been gradually removed with Wine commits:
  https://github.com/wine-mirror/wine/commit/c99754ef15a8e5bc0fd9189fc81e4e4d9cc1f589
  https://github.com/wine-mirror/wine/commit/6b889fe9188a97765bcbcc2cae5ca2480a73e7af
  https://github.com/wine-mirror/wine/commit/5cd6db03495d73e3db024472514a753a44a06772
  https://github.com/wine-mirror/wine/commit/26c9bd9f15c364215be9731bb050454c14d90767
Now, a mere `ERR("Wine Mono is not installed\n");` is emitted.

dll/win32/mscoree/metahost.c

@@ -82,6 +82,19 @@ static void CDECL do_nothing(void)
 {
 }
 
+#ifdef __REACTOS__
+int WINAPIV ShellMessageBoxA(HINSTANCE hAppInst, HWND hWnd, LPCSTR lpcText, LPCSTR lpcTitle, UINT fuStyle, ...);
+#undef MESSAGE
+#define MESSAGE(msg) \
+do { \
+    WINE_MESSAGE((msg)); \
+    ShellMessageBoxA(NULL, NULL, (msg), "Wine Mono", MB_OK | MB_ICONSTOP); \
+} while(0)
+//
+// NOTE for wine-syncs: This warning is gradually removed in Wine commits:
+// c99754ef15a8, 6b889fe9188a, 5cd6db03495d, and 26c9bd9f15c3
+//
+#endif
 static void missing_runtime_message(const CLRRuntimeInfo *This)
 {
     if (This->major == 1)