fixed segfaults caused by preset_hub_read and preset_hub_readpreset (may want to revisit later to disable the crflag from the binbuf_read_via_canvas and consider implications of potentially running into an A_SEMI which currently does not appear to be likely, or dare I say, possible?)

This is needed to build when some dependencies are installed into
locations outside of /usr.

copied GNU/Linux distro installation instructions from https://github.com/agraef/purr-data/wiki/Installation, and removed references between instructions for different platforms

This commit adds initial packaging to build and distribute Purr Data
using Flatpak.

This brings us in line with the [build
API](https://github.com/cgwalters/build-api) used by flatpak-builder.