Fails to link against system picojson

Hi, I wanted to package this plugin for debian.
Linking against the system version of picojson fails though:

g++ -static-libgcc -fPIC -static-libstdc++ -shared -o libcallex.so libcallex.cxx -lffi
libcallex.cxx: In function ‘const char* libcallex_call(const char*)’:
libcallex.cxx:146:40: error: no match for ‘operator=’ (operand types are ‘std::map<std::__cxx11::basic_string<char>, picojson::value>::mapped_type’ {aka ‘picojson::value’} and ‘std::__cxx11::basic_stringstream<char>::__string_type’ {aka ‘std::__cxx11::basic_string<char>’})
  146 |                 obj["return"] = ss.str();
      |                                        ^
In file included from libcallex.cxx:9:
/usr/include/picojson.h:329:15: note: candidate: ‘picojson::value& picojson::value::operator=(const picojson::value&)’
  329 | inline value &value::operator=(const value &x) {
      |               ^~~~~
/usr/include/picojson.h:329:45: note:   no known conversion for argument 1 from ‘std::__cxx11::basic_stringstream<char>::__string_type’ {aka ‘std::__cxx11::basic_string<char>’} to ‘const picojson::value&’
  329 | inline value &value::operator=(const value &x) {
      |                                ~~~~~~~~~~~~~^
/usr/include/picojson.h:341:15: note: candidate: ‘picojson::value& picojson::value::operator=(picojson::value&&)’
  341 | inline value &value::operator=(value &&x) PICOJSON_NOEXCEPT {
      |               ^~~~~
/usr/include/picojson.h:341:40: note:   no known conversion for argument 1 from ‘std::__cxx11::basic_stringstream<char>::__string_type’ {aka ‘std::__cxx11::basic_string<char>’} to ‘picojson::value&&’
  341 | inline value &value::operator=(value &&x) PICOJSON_NOEXCEPT {
      |                                ~~~~~~~~^
libcallex.cxx:147:21: error: no match for ‘operator=’ (operand types are ‘picojson::value’ and ‘picojson::object’ {aka ‘std::map<std::__cxx11::basic_string<char>, picojson::value>’})
  147 |                 v = obj;
      |                     ^~~
/usr/include/picojson.h:329:15: note: candidate: ‘picojson::value& picojson::value::operator=(const picojson::value&)’
  329 | inline value &value::operator=(const value &x) {
      |               ^~~~~
/usr/include/picojson.h:329:45: note:   no known conversion for argument 1 from ‘picojson::object’ {aka ‘std::map<std::__cxx11::basic_string<char>, picojson::value>’} to ‘const picojson::value&’
  329 | inline value &value::operator=(const value &x) {
      |                                ~~~~~~~~~~~~~^
/usr/include/picojson.h:341:15: note: candidate: ‘picojson::value& picojson::value::operator=(picojson::value&&)’
  341 | inline value &value::operator=(value &&x) PICOJSON_NOEXCEPT {
      |               ^~~~~
/usr/include/picojson.h:341:40: note:   no known conversion for argument 1 from ‘picojson::object’ {aka ‘std::map<std::__cxx11::basic_string<char>, picojson::value>’} to ‘picojson::value&&’
  341 | inline value &value::operator=(value &&x) PICOJSON_NOEXCEPT {
      |                                ~~~~~~~~^
libcallex.cxx:165:40: error: ‘picojson::value::value(const T*) [with T = char]’ is private within this context
  165 |                 obj["error"] = dlerror();
      |                                        ^
/usr/include/picojson.h:202:25: note: declared private here
  202 |   template <typename T> value(const T *); // intentionally defined to block implicit conversion of pointer to bool
      |                         ^~~~~
make[2]: *** [Makefile:5: libcallex.so] Error 1
make[2]: Leaving directory '/<<PKGBUILDDIR>>/autoload'
make[1]: *** [debian/rules:11: execute_after_dh_autoreconf] Error 2
make[1]: Leaving directory '/<<PKGBUILDDIR>>'
make: *** [debian/rules:7: binary] Error 2

The version of picojson included in the repo is really old; so I would appreciate support for linking against the system picojson.

Start with libcallex.cxx at the reported assignments around lines 146-165 and compare them with the system picojson API shown in the compiler errors. Check the Makefile build target and reproduce the Debian build using the system header; done means libcallex.so builds successfully without the bundled old picojson.