Build fails when GL=ON

[yurivict]

/usr/ports/audio/guidolib/work/guidolib-177/environments/opengl/device/GSystemGL.cpp:127:13: error: allocating an object of abstract class type 'GFontGL'
        return new GFontGL( font, faceName, size, properties, fSymbolTable );
                   ^
/usr/ports/audio/guidolib/work/guidolib-177/build/../src/engine/include/VGFont.h:68:18: note: unimplemented pure virtual method 'GetExtent' in 'GFontGL'
                virtual void                    GetExtent( int c,
                                                ^
1 error generated.

Version: 177
clang-16
FreeBSD 13.2