Add GNU visibility generation and ABI check script

It seems that all the library projects have their own copies of this, which seems kinda silly. And there are some downsides to those files:

• The visibility alias file generation script is written in perl. This isn't generally a bad thing, but requires people have perl installed. With the meson build, python will be required, so might as well write it in python.
• The symbols file consumed by the scripts is a C preprocessor file, and its a bit tedious, annoying, and error-prone to write. The new format that the new scripts consume is simple and easy to read and write.

There was also never really any documentation for how to set all this up, so I've written a README that describes the process for both autotools and meson.