Found the following significant warnings:
  libjson/Source/JSONGlobals.h:43:50: warning: invoking macro json_global_decl argument 3: empty macro arguments are undefined in ISO C++98 [-Wpedantic]
  libjson/Source/JSONGlobals.h:44:49: warning: invoking macro json_global_decl argument 3: empty macro arguments are undefined in ISO C++98 [-Wpedantic]
  libjson/Source/JSONSharedString.h:297:17: warning: ‘json_shared_string::len’ will be initialized after [-Wreorder]
  libjson/Source/JSONSharedString.h:295:40: warning:   ‘json_shared_string::json_shared_string_internal* json_shared_string::_str’ [-Wreorder]
  libjson/Source/JSONSharedString.h:180:9: warning:   when initialized here [-Wreorder]
  libjson/Source/JSONSharedString.h:182:9: warning:   when initialized here [-Wreorder]
  libjson/Source/JSONStream.h:79:7: warning: ‘JSONStream::state’ will be initialized after [-Wreorder]
  libjson/Source/JSONStream.h:76:28: warning:   ‘void (* JSONStream::call)(void*, void*)’ [-Wreorder]
  JSONStream.cpp:8:1: warning:   when initialized here [-Wreorder]
  libjson/Source/JSONStream.h:77:27: warning: ‘JSONStream::err_call’ will be initialized after [-Wreorder]
  libjson/Source/JSONStream.h:75:17: warning:   ‘json_string JSONStream::buffer’ [-Wreorder]
  JSONStream.cpp:12:1: warning:   when initialized here [-Wreorder]
  JSONValidator.cpp:394:105: warning: invoking macro JSON_ASSERT_SAFE argument 3: empty macro arguments are undefined in ISO C++98 [-Wpedantic]
  JSONValidator.cpp:397:104: warning: invoking macro JSON_ASSERT_SAFE argument 3: empty macro arguments are undefined in ISO C++98 [-Wpedantic]
  libjson/Source/internalJSONNode.h:257:18: warning: ‘internalJSONNode::_name_encoded’ will be initialized after [-Wreorder]
  libjson/Source/internalJSONNode.h:256:17: warning:   ‘json_string internalJSONNode::_name’ [-Wreorder]
  internalJSONNode.cpp:78:1: warning:   when initialized here [-Wreorder]
See ‘/home/hornik/tmp/R.check/r-patched-gcc/Work/PKGS/RJSONIO.Rcheck/00install.out’ for details.