When the "unstable" Cargo feature is given, html5ever’s build script
will use html5ever_macros as a library to generate or update
src/tree_builder/rules.expanded.rs, which is part of the source tree.

(This feature also enables the corresponding features of tendril and
string-cache.)

html5ever_macros is not a plugin anymore, but it still uses libsyntax
and still behaves a lot like a syntax extension.

When "unstable" is *not* given, the build script checks that
rules.expanded.rs is up to date (based on a hash of rules.rs) and fails
if it’s not.