Ignore failures on removing ext sub directories

When multiple libraries exist in a subdirectory under `ext`, `rmdir
-p` may fail, because other directories have not been removed yet or
the parent directory has been removed by other `distclean`.  `rmdir`
in GNU coreutils has `--ignore-fail-on-non-empty` option for the
former case but others may not, and the latter race condition is not
avoidable anyway.

template/exts.mk.tmpl

@@ -162,7 +162,7 @@ ext/extinit.<%=objext%>:
 	$(Q)<%= submake %><%=mflags%> V=$(V) $(@F)
 %     if /^(dist|real)clean$/ =~ tgt
 	$(Q)$(RM) <%=t[%r[\A(?:\.[^/]+/)?(?:[^/]+/){2}]]%>exts.mk
-	$(Q)$(RMDIRS) $(@D)
+	-$(Q)$(RMDIRS) $(@D)
 %     end
 %   end
 % end