configure: Add a 'Generate files' section

This refactor groups together the file generation into a single
section.
configure: Add TOC entry for --enable-authentication

Just the comments, not the code.

Issue #176.
configure: Fix minor inaccuracy in help string
configure: Add a --enable-authentication argument

Currently it doesn't actually do anything, it's just the argument.

Issue #176.
configure: Add authentication requirement to .htaccess

... if --enable-authentication is enabled, of course.

Issue #176.
configure: Add htpasswd env variables

You can pass the authentication details during configure invocation:

    $ ./configure --enable-authentication \
            SITE_USERNAME=foo SITE_PASSWORD=bar

Issue #176.
configure: Generate www/.htpasswd

Issue #176.
build/www: Export and install .htpasswd

... if authentication is enabled.

Issue #176.