overriding default value of localstatedir

[john heasley]

I'd like to override the default value of localstatedir (in a distribution).
That is, unless specifically defined with configure --localstatedir, I'd
like the value to be /var/packagename.

AFAICT, autoconf does not offer a way to do this.  I can override the value
after AC_INIT(), but <prefix>/var still appears in the --help output and
there is no easy way to determine if it had been specified on the command-line.

Am I missing some knob to acheive this?