Repair Mac Build processes

[mackworth]

[FIX]

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Looks like the MacOS build processes got left behind with some recent changes (e.g. moving zvbi).

After much travail, I've updated several components, and it now build fine through all three mechanisms. (Although not sure why you're keeping the build.command around, given the other mechanisms are much more flexible.).

Notes below, but in particular, I've added the libpng NEON files necessary for building for the Apple Silicon Macs, and I've made the autoconf files identical for Mac and Linux (which fixed many of the Mac issues). Please let me know if you disagree with any of these changes. Happy to update this PR.

I should note that I have not tried building with rust yet at all. Is it materially better yet than the C version?

For all:
Add Neon files to libpng for Apple Silicon
Update compilation.md documentation

For autoconf:
Make Linux and Mac Makefile.am and configure.ac identical
Fix wrong location for zvbi/bcd.h in both Mac/Linux

For cmake:
Include GPAC config for Darwin in Mac version

For mac/build.command:
Update for new zvbi location