When building on Debian/Ubuntu with install prefix /usr (e.g. in PPA builds) the CMAKE_INSTALL_LIBDIR is resolved to lib/x86_64-linux-gnu. For jsoncpp external project this is never the case because the install prefix is not /usr. Remove multiarch part from libdir if there.
This copies jsoncpp.cmake from ethereum/cpp-dependencies repo and adds BUILD_BYPRODUCT information to ExternalProject configuration (required by Ninja).