Fixes for most compiler warnings using GCC 7.3.0 and mingw-w64.

[JPeterMugaas]

patching file src/config.h Add an UNUSED macro to set the unused gcc attribute to fix compiler warnings patching file src/fileio.hpp Fix unused parameter warnings with UNUSED macro patching file src/flif.cpp Fix unused parameter warnings with UNUSED macro patching file src/flif-dec.cpp Fix unused parameter warnings with UNUSED macro patching file src/flif-enc.cpp Fix unused parameter warnings with UNUSED macro patching file src/viewflif.c add a copy of the UNUSED macro here so the example will compile without warnings fix warnings about an invalid format string. mingw-w64 uses something different than MS Visual C. Fix sign-comparison warnings between uint32_t and int. Some of that is because the SDL API uses int while FLIF uses uint32_t. patching file src/image/image.hpp Fix unused parameter warnings with UNUSED macro Add "falls through" comment to some case statements to eliminate GCC warnings about fallthrough. patching file src/library/flif_common.h MINGW32 also requires dllimport/dllexport patching file src/library/flif-interface-private_common.hpp MINGW32 also requires dllexport patching file src/maniac/compound.hpp Fix unused parameter warnings with UNUSED macro patching file src/maniac/rac_enc.hpp Fix unused parameter warnings with UNUSED macro patching file src/transform/colorbuckets.hpp Fix unused parameter warnings with UNUSED macro patching file src/transform/framecombine.hpp Fix unused parameter warnings with UNUSED macro patching file src/transform/transform.hpp Fix unused parameter warnings with UNUSED macro patching file src/transform/palette_C.hpp Fix unused parameter warnings with UNUSED macro patching file src/transform/ycocg.hpp Fix unused parameter warnings with UNUSED macro

[jonsneyers]

Thanks!