Aside from including a ton of bug fixes, this should fix an issue in the
CI pipeline when building with system libraries that was caused by a
minor incompatibility between 0.5.0 and less ancient versions.
Also, switch to the amalgamated source, because it makes it easier to
keep it up to date.
https://github.com/open-source-parsers/jsoncpp/wiki/Amalgamated-(Possibly-outdated)
Martin Natano