add source for "racket-playsound.exe"

racket/src/native-libs/README.txt

@@ -155,6 +155,9 @@ More details for Windows:
    uses `-static-libgcc' and `-static-libstdc++' to statically link
    those libraries. Use "depends.exe" to check DLL dependencies.
 
+ * The extra "playsound.c" program is meant to be compiled to
+   "racket-playsound.exe" for the "gui" library.
+
 More details for Mac OS X:
 
  * 32-bit binaries are built for 10.5 and up. 64-bit binaries are

racket/src/native-libs/playsound.c

@@ -0,0 +1,10 @@
+#include <Windows.h>
+
+/* Doesn't attempt to parse the command line in the usual way.
+   Instead, the whole command-line is used as a path of a
+   sound file to play. */
+
+int APIENTRY WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, LPSTR ignored, int nCmdShow)
+{
+  return PlaySoundW(GetCommandLineW(), NULL, SND_SYNC);
+}