warning: ISO C++11 does not allow conversion from string literal to 'char *' [-Wwritable-strings]

[ryandesign]

When compiling GLE 4.3.4 with clang this warning appears:

src/gle/tex.cpp:1569:31: warning: ISO C++11 does not allow conversion from string literal to 'char *' [-Wwritable-strings]
        if (c < 0 || c > 255) return "";
                                     ^
1 warning generated.

The problem code is here:

https://github.com/vlabella/GLE/blob/97cf2ca024ef9a12abf6fe6024439faf0c82a0e8/src/gle/tex.cpp#L1568-L1571