configure.in: note to remove a hack

* configure.in (rm): note to remove this hack when AC_PREREQ(2.70) or
  later.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39648 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

configure.in

@@ -13,6 +13,8 @@ AC_DISABLE_OPTION_CHECKING
 
 # suppress error messages, rm: cannot remove 'conftest.dSYM', from
 # AC_EGREP_CPP with CFLAGS=-g on Darwin.
+#
+# TODO: remove this hack when AC_PREREQ() becomes 2.70 or later.
 rm() {
     local recursive
     for arg do