`./configure --help` fails if `configure` is dislocated

[Werner LEMBERG]

Take any `configure` script and move it to another directory.  For
example, executing the script from Emacs gives

```
configure: error: cannot find sources (src/lisp.h) in . or ..
```

which is ok.  However, and I consider this a bug, you get exactly the
same error message if you say `./configure --help`.  In other words,
the user has no chance to find out that there exists a `--srcdir`
option to override `AC_CONFIG_SRCDIR`.

I thus suggest to either make `./configure --help` always work, or at
least change the above error message to something like

```
configure: error: cannot find sources (src/lisp.h) in . or ..
                  Are you missing a `--srcdir=DIR` command line option?
```

It is probably another bug that a line like

```
AC_CONFIG_HEADERS(src/config.h:src/config.in)
```

fails with

```
config.status: error: cannot find input file: `src/config.in'
```

inspite of a `--srcdir` option...


    Werner