Unknown ``GifErrorString``

The function GifErrorString is only available in giflib version 5, as discussed here: http://lists.openimageio.org/pipermail/oiio-dev-openimageio.org/2013-October/006315.html

On my machine (with giflib version 4.1.6-11) I get the compilation error:

bob.io.image/bob/io/image/cpp/gif.cpp: In function ‘void GifErrorHandler(const char*, int)’:
bob.io.image/bob/io/image/cpp/gif.cpp:284:50: error: ‘GifErrorString’ was not declared in this scope
   const char* error_string = GifErrorString(error);

@andre.anjos Could you please fix that ASAP? The master branch is not compiling on my machine any more.