Makefile: Allow "make clean" to delete compat/atomics/pthread/stdatomic.o

Reported-by: Eric Thomas

Makefile

@@ -135,7 +135,7 @@ uninstall-data:
 
 clean::
 	$(RM) $(CLEANSUFFIXES)
-	$(RM) $(addprefix compat/,$(CLEANSUFFIXES)) $(addprefix compat/*/,$(CLEANSUFFIXES))
+	$(RM) $(addprefix compat/,$(CLEANSUFFIXES)) $(addprefix compat/*/,$(CLEANSUFFIXES)) $(addprefix compat/*/*/,$(CLEANSUFFIXES))
 	$(RM) -r coverage-html
 	$(RM) -rf coverage.info coverage.info.in lcov