Build without Sparkle, for app store distribution

This is a step towards preparing for a Mac app store distribution.

declare a new target, whatdid-app-store; this is like whatdid-release, but without the Sparkle library
add a bunch of #if canImports to conditionally do Sparkle-related stuff depending on whether we have it.
matrix-ify the build, so that we can sanity check that it still compiles with Sparkle removed

yshavit / whatdid