[patch] use python-config if available

[Joel Brobecker]

> The patch is fine with me.
> [The plan wasn't to shift and call the "real" python-config ... at the
> time there was no real python-config for cross builds.]
> 
> One typo: should be ${python_config}
> 
> -        AC_ERROR(failure running python-config --exec-prefix)
> +        AC_ERROR(failure running ${python-config} --exec-prefix)

Can we make this new behavior optional? The "real" python-config
is broken for several platforms, and having our own that we can
control was very convenient.

-- 
Joel