Sort input file list

[bmwiedemann]

Sort input file list
so that epub2txt builds in a reproducible way
in spite of indeterministic filesystem readdir order

See https://reproducible-builds.org/ for why this is good.

This PR was done while working on reproducible builds for openSUSE.

[kevinboone]

I'm not sure that anything in the source is going to be affected by the compilation or link order; still, it seems a reasonable change. Thanks.

[bmwiedemann]

For the record: The link order of $(OBJECTS) does influence the resulting binary, at least with gcc+binutils.

[kevinboone]

Sure; but does it affect whether something works, or does not work?

[bmwiedemann]

It might work the same way, but a) there is no guarantee or proof and b) reproducible builds aims at bit-identical results, so you can tell people this one hash value is the expected good one (when building with a given build env).