CMake defaults to C *and* C++ toolchain, in case nothing has been specified.
This means that cmake always checks for both, which is more than needed.
This PR cuts off C toolchain requirement from /CMakeLists.txt and ensures that we
don't pass along any `..._C_...` variables in EthCompilerSettings.cake nor jsoncpp.cmake.
According to cmake documents, we cannot assume CMAKE_INSTALL_LIBDIR is a
relative path. This commit fixes the "no rule to make libjsoncpp.a"
error by passing -DCMAKE_INSTALL_LIBDIR=lib to jsoncpp external project.
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).