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 !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.