configure: Number each of the section headings
lib: Generate config.php at configure time

Issue #164.
build: Allow install when WWW_ROOT doesn't exist

There are valid circumstances under which WWW_ROOT doesn't yet exist,
but we still want to install anyway, so instead just decide whether to
install based on if ENABLE_INSTALL is set.

Issue #164.
config.php: Use WWW_PREFIX for calculating links

Issue #164.
configure: Always export WWW_ROOT

... not just if we find that the file exists

Issue #164.
configure: Add database credentials variables

We can now set our database credentials at configure time:

    $ ./configure DB_NAME=foo DB_PASSWORD=foobar

Issue #164.