Add better handling when deleting files on Windows

Windows complains if you try to delete a file that doesn't exist.
Makefiles now check if the files exist before trying to delete them.
diff --git a/programs/Makefile b/programs/Makefile
index 9cc28c4..f3627c9 100644
--- a/programs/Makefile
+++ b/programs/Makefile
@@ -322,7 +322,9 @@
 ifndef WINDOWS
 	rm -f $(APPS) $(EXTRA_GENERATED)
 else
-	del /S /Q /F *.o *.exe $(EXTRA_GENERATED)
+	if exist *.o del /S /Q /F *.o
+	if exist *.exe del /S /Q /F *.exe
+	if exist $(EXTRA_GENERATED) del /S /Q /F $(EXTRA_GENERATED)
 endif
 
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