[libpng18] spurious autoconf files

[jbowler]

scripts/autoconf remains even though the rest of the autoconf files have been deleted, this causes really weird behavior.

[ctruta]

Ha! This:

libpng/autogen.sh

Line 84 in 00343a7

# These are separate because 'maintainer-clean' does not remove them.

So I missed them.

[ctruta]

Fixed.

[ctruta]

... and I missed your commit ff3f84f, which does this also.
In any case, I'll send an announcement to png-mng-implement, and I'll start picking up commits.

[jbowler]

I'll start picking up commits.

Yeah; I layered the autoconf fix on all the other changes since I needed it to continue testing. I just revalidated the use-ISO-... fix so the now rebased #575, #589, #590 sequence are good to go. I've tested on Intel SSE and AArch64 (before the pngvalid/pngstest breaks).

I still don't know exactly how they got broken; I'm pretty sure I can find it but I don't want to do that if you are going to remove all the libpng 1.7 #ifs en masse.