Handle deleting non-existant files on Windows

If we try to delete a non-existant file using del on Windows, as
can happen when running make clean, del will throw an error. Make
the Makefiles more robust by only deleting files if they exist.

library/Makefile

@@ -165,5 +165,7 @@ clean:
 ifndef WINDOWS
 	rm -f *.o libmbed* $(OBJS_CRYPTO)
 else
-	del /Q /F *.o libmbed* $(OBJS_CRYPTO)
+	if exist *.o del /Q /F *.o
+	if exist libmbed* del /Q /F libmbed*
+	if exist $(OBJS_CRYPTO) del /Q /F $(OBJS_CRYPTO)
 endif

programs/Makefile

@@ -317,7 +317,8 @@ ifndef WINDOWS
 	-rm -f ssl/ssl_pthread_server$(EXEXT)
 	-rm -f test/cpp_dummy_build$(EXEXT)
 else
-	del /S /Q /F *.o *.exe
+	if exist *.o del /Q /F *.o
+	if exist *.exe del /Q /F *.exe
 endif
 	$(MAKE) -C fuzz clean
 

programs/fuzz/Makefile

@@ -69,5 +69,6 @@ clean:
 ifndef WINDOWS
 	rm -rf $(BINARIES) *.o
 else
-	del /Q /F *.o *.exe
+	if exist *.o del /Q /F *.o
+	if exist *.exe del /Q /F *.exe
 endif

tests/Makefile

@@ -160,7 +160,9 @@ clean:
 ifndef WINDOWS
 	rm -rf $(BINARIES) *.c *.datax TESTS
 else
-	del /Q /F *.c *.exe *.datax
+	if exist *.c del /Q /F *.c
+	if exist *.exe del /Q /F *.exe
+	if exist *.datax del /Q /F *.datax
 ifneq ($(wildcard TESTS/.*),)
 	rmdir /Q /S TESTS
 endif