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.

This gets rid of some undefined behavior that occurs from sending an
int as an argument to dsp_add which reads it back as a t_int. On certain
archs this will produce garbage values in the perform routine which can
cause crashes.

Obs detected bugs

See merge request jwilkes/purr-data!313

Obs detected bugs, 3rd stage

See merge request jwilkes/purr-data!316

Allow make options like -j8 to be passed to the Gem compilation, which takes awfully long on a single cpu.

Obs detected bugs, 2nd stage

See merge request jwilkes/purr-data!314