bootstrap: Restore support for system jsoncpp and uv without pkg-config

In commit da5de7f9f3 (bootstrap: Allow --boostrap-system-* libraries
custom prefixes, 2024-03-03, v3.30.0-rc1~456^2) the non-pkg-config code
path for uv/jsoncpp/rhash all set `use_librhash_ldflags` instead of
their own variable.