Bug#828890: libsdl2-gfx: please make the build reproducible

[Gianfranco Costamagna]

control: tags -1 pending

>While working on the "reproducible builds" effort [1], we have noticed
>that libsdl2-gfx could not be built reproducibly.>The example tarball still has time and order variations.
>
>The attached patch fixes this by telling tar to sort the file list and
>clamp the mtime at SOURCE_DATE_EPOCH.



*thanks* :)
uploaded with some other changes!
thanks again for the great help!

Gianfranco