Use MiniPortile2's mkmf_config to simplify build

GitHub: #109

Now MiniPortile2 2.8.5 ships with a mkmf_config feature that can handle
statically linking libraries for us, switch to using that rather than
rolling our own implementation.

At the same time, we take a leaf from the Ruby SQLite3 gem's extconf
(https://github.com/sparklemotion/sqlite3-ruby/blob/ae5c13996f936fce07e8a5e9fb6aaf2ede5d82b7/ext/sqlite3/extconf.rb#L113)
and re-organise the configuration logic in our extconf.rb into a class
rather than a series of global functions.