Fails to build with giflib 5

[fxcoudert]

The fontforge branch bundled with pdf2htmlEX does not build against the current giflib versions, i.e. giflib 5 and later. The differences in giflib 5 API are explained there: http://giflib.sourceforge.net/gif_lib.html#compatibility

[fxcoudert]

The problem is with occurrences of DGifCloseFile(gif) in gutils/gimagereadgif.c, which need to be replaced with:

#ifdef _GIFLIB_51PLUS
        DGifCloseFile(gif, NULL);
#else
        DGifCloseFile(gif);
#endif