clang 3.8 : compile fail

[proller]

src/lib_json/json_tool.h:66:18: error: implicit conversion changes signedness: 'signed char' to 'char' [-Werror,-Wsign-conversion]
*--current = static_cast(value % 10U + static_cast('0'));
~ ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

[cdunn2001]

Please re-open if this change does not solve the problem.