re-added GCC6
re-added GCC6

Newbrick/json,nlohmann/json,hunter-packages/json,hunter-packages/json,nlohmann/json,whackashoe/json,theodelrieu/json,kaidokert/json,stanmihai4/json,stanmihai4/json,Newbrick/json,stanmihai4/json,hunter-packages/json,kaidokert/json,Newbrick/json,whackashoe/json,kaidokert/json,nlohmann/json,theodelrieu/json