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.

library/Makefile

@@ -200,5 +200,6 @@ clean:
 ifndef WINDOWS
 	rm -f *.o libmbed*
 else
-	del /Q /F *.o libmbed*
+	if exist *.o del /Q /F *.o
+	if exist libmbed* del /Q /F libmbed*
 endif