WinRT: ugh, at least one file in an app's project seems to require C++/CX compilation.

Assuming this is true, that file might as well be the one that contains WinMain.

src/main/winrt/SDL_winrt_main_NonXAML.cpp

@@ -1,6 +1,33 @@
 
 #include <wrl.h>
 
+/* At least one file in any SDL/WinRT app appears to require compilation
+   with C++/CX, otherwise a Windows Metadata file won't get created, and
+   an APPX0702 build error can appear shortly after linking.
+
+   The following set of preprocessor code forces this file to be compiled
+   as C++/CX, which appears to cause Visual C++ 2012's build tools to
+   create this .winmd file, and will help allow builds of SDL/WinRT apps
+   to proceed without error.
+
+   If other files in an app's project enable C++/CX compilation, then it might
+   be possible for SDL_winrt_main_NonXAML.cpp to be compiled without /ZW,
+   for Visual C++'s build tools to create a winmd file, and for the app to
+   build without APPX0702 errors.  In this case, if
+   SDL_WINRT_METADATA_FILE_AVAILABLE is defined as a C/C++ macro, then
+   the #error (to force C++/CX compilation) will be disabled.
+
+   Please note that /ZW can be specified on a file-by-file basis.  To do this,
+   right click on the file in Visual C++, click Properties, then change the
+   setting through the dialog that comes up.
+*/
+#ifndef SDL_WINRT_METADATA_FILE_AVAILABLE
+#ifndef __cplusplus_winrt
+#error SDL_winrt_main_NonXAML.cpp must be compiled with /ZW, otherwise build errors due to missing .winmd files can occur.
+#endif
+#endif
+
+
 /* The app's C-style main will be passed into SDL.dll as a function
    pointer, and called at the appropriate time.
 */