Advantage of libcurl over wxHTTP is that we get HTTPS support out of the box.
Disadvantage is it being another dependency. Therefore using libcurl is optional
now, but strongly encouraged.

Since HTTP download was the last piece of network functions not replaced by
Boost.Asio, using libcurl together with boost makes it possible to forget wxWidgets
networking completely.

Minimum required libcurl version is now 7.32.0, required by CURLOPT_XFERINFOFUNCTION.
Since it's already seven years old, we're not planning to support older versions.

This is mainly a demonstration of how we can select new functionality if available.
First, we have to check the compile-time headers version whether they have the new
functionality or not, to be able to compile it in. Then, we have to check the library
version at runtime because that might very well be different from what we used for
compilation. If both checks succeed then and only then can we use the new interface.
In all other cases we need to revert to use the old interface.

We use the progress callback to periodically check whether the current download
is requested to be cancelled (i.e. the user pressed the 'Cancel' button, or the
application is exiting). That requires that we always have a progress callback
installed, therefore the minimum required libcurl version is lowered to 7.19.1.