[pdftex] Compiling on M1 Mac

[Karl Berry]

Hi Jason - I synced the pdftex repo from tex live, which I think will
get the latest libpng. If that works, great.

Another possibility is to run
  ./build-pdftex --enable-arm-neon=yes
I believe the command line arguments will be passed down to (all the)
subconfigures, and dutifully ignored except by libpng.

Another possibility is to try compiling tl. It's easiest to grab the
entire source tree, but then it's somewhat straightforward, if annoying,
to disable compilation of everything but a given engine:
https://tug.org/texinfohtml/tlbuild.html#Build-one-engine

Hope something in there works.  The whole arm/neon thing has been
nothing but trouble.

Thanks for the report. -k