configure --help & envvars

[karl]

The end of configure --help says:

    Some influential environment variables:
       ...
    Use these variables to override the choices made by `configure' or to help
    it to find libraries and programs with nonstandard names/locations.

I suggest mentioning here again at the end to set these with configure
ENVVAR=VALUE instead of with the usual ENVVAR=VALUE configure.  That
little blurb at the top about this is a long ways away :).

Just a suggestion,
karl