Also build a static library for other tools to link against.
Make Gif2UnicornHat more friendly to use as a library.
 - Rename the namespace to Gif2UnicornHat
 - Move complicated signal handling logic off into its own compilation unit.
Exclude main.o from being archived into the library so main() doesn't collide with whoever links against it.
Fix shutdown logic to be more sure we are calling terminate() no matter how the process exits.