Compiler warning: ISO C++ forbids converting a string constant to 'char*'

[iansan5653]

I originally opened this in the upstream repo, but it looks like this fork is the most maintained one so I am copying it here instead.

The library compiles, but several instances of this warning are output to the console:

In file included from d:\Git Repositories\knotmeter\knotmeter\code\main.ino:3:0:
C:\Users\Ian Sanders\Documents\Arduino\libraries\HT1621\src/HT1621.h:60:35: warning: ISO C++ forbids converting a string constant to 'char*' [-Wwrite-strings]
void print(long num, char* flags="%6li", int precision = 0);
^~~~~~
In file included from C:\Users\Ian Sanders\Documents\Arduino\libraries\HT1621\src\HT1621.cpp:33:0:
C:\Users\Ian Sanders\Documents\Arduino\libraries\HT1621\src\HT1621.h:60:35: warning: ISO C++ forbids converting a string constant to 'char*' [-Wwrite-strings]
void print(long num, char* flags="%6li", int precision = 0);
^~~~~~
C:\Users\Ian Sanders\Documents\Arduino\libraries\HT1621\src\HT1621.cpp: In member function 'void HT1621::print(double, int)':
C:\Users\Ian Sanders\Documents\Arduino\libraries\HT1621\src\HT1621.cpp:241:17: warning: ISO C++ forbids converting a string constant to 'char*' [-Wwrite-strings]
char * flags = "%6li";
^~~~~~
C:\Users\Ian Sanders\Documents\Arduino\libraries\HT1621\src\HT1621.cpp:244:11: warning: ISO C++ forbids converting a string constant to 'char*' [-Wwrite-strings]
flags = "%06li";
^~~~~~~
C:\Users\Ian Sanders\Documents\Arduino\libraries\HT1621\src\HT1621.cpp: In member function 'void HT1621::printCelsius(double)':
C:\Users\Ian Sanders\Documents\Arduino\libraries\HT1621\src\HT1621.cpp:270:17: warning: ISO C++ forbids converting a string constant to 'char*' [-Wwrite-strings]
char * flags = "%4li*C";
^~~~~~~~
C:\Users\Ian Sanders\Documents\Arduino\libraries\HT1621\src\HT1621.cpp:273:11: warning: ISO C++ forbids converting a string constant to 'char*' [-Wwrite-strings]
flags = "%04li*C";
^~~~~~~~~