configure: Make the printed summary easy to understand (hopefully!)

Make the summary that is printed at the end executing the configure
script easier to read and understand.

Issue #160.
configure: Refactor --enable-httpd to --enable-install

So it turns out there already was a configure argument to enable/disable
installing the built webserver. The fact that I'd forgotten all about it
and didn't spot it in the help output shows that it clearly didn't have
an obvious name, so rename it 'enable-install'.

Issue #162.
configure: Remove an unused section

It's been on the TODO list for too long, and clearly isn't something
that seems important. Remove.

Issue #160.
configure: Don't set a traditional $PREFIX

This would only confuse matters. We don't care about the $PREFIX because
we set all of our own.

Issue #160.
configure: Print webserver summary if we are enabling install

There are valid circumstances under which we want to install the built
website, but the $WWW_ROOT doesn't exist. In that case, using
$HAVE_WWW_ROOT as the conditional isn't appropriate.

Issue #160.
configure: Print whether we can build md TOCs

This is useful to know.

Issue #160.
configure: Über overhaul!

The aim here is to reorder the configure script so that it makes logical
sense. This doesn't actually change the functionality, just the order of
execution.

Issue #160.
configure: Refactor 'extra-tools' to just 'tools'

There's no need to consider them 'extra'.

Issue #160.
configure: Refactor 'extra-documentation' to 'documentation'

There's no need to consider it 'extra'.

Issue #160.
configure: Only build docs if --enable-documentation

Issue #160.
configure: Only check for tools dependencies if enabled

If we're not building the tools, then we don't need to check for the
dependencies (node, npm, optimist).

Issue #160.
configure: Macro expansion fixes

Issue #160.
configure: Enable everything by default

We should only have to use the --disable-[feature] options to override
these defaults.

Issue #60.
configure: Perform git config last

This re-orders the configure execution so that the git hooks
configuration is run last.

Issue #160.