Compilation throws "forbidden conversion" warnings

[rdupas]

g++ version: g++ (Debian 6.3.0-18+deb9u1) 6.3.0 20170516
pdf2json version: 0.70
chunck of the trace:

ImgOutputDev.cc:833:66: warning: ISO C++ forbids converting a string constant to 'char*' [-Wwrite-strings]
   glMetaVars->append(new HtmlMetaVar("generator", "pdf2json 0.70"));
                                                                  ^
ImgOutputDev.cc:833:66: warning: ISO C++ forbids converting a string constant to 'char*' [-Wwrite-strings]
ImgOutputDev.cc:834:67: warning: ISO C++ forbids converting a string constant to 'char*' [-Wwrite-strings]
   if( author ) glMetaVars->append(new HtmlMetaVar("author", author));
                                                                   ^