I have a string that I want to use multiple times for the output of `configure --help`. So I try doing something like this in `configure.ac`:

```
AC_ARG_ENABLE([foobar],
              AS_HELP_STRING([--enable-foobar], [$foobar_help_str]))

```

But no expansion or substitution is done, so the output is just `$foobar_help_str`.