warning: inline variables are only available with ‘-std=c++17’ or ‘-std=gnu++17’ how to suppress it?

Using below function in one of the .h file. if i am using function deceleration without inline getting multiple definitions error and now after adding inline getting above warning so help me to suppress the warning inline void (*log_fcn)(int level, std::string format, unsigned int line_no, std::string file_name, ...);.