Add xcconfig file for generate_appcast target

[zorgiepoo]

Also remove swift configurations from Common config files, and change swift optimization from fast to whole module.

For some reason unknown to me, generate_appcast fails to build in Xcode 'All' or 'Distribution' target. This can be reproduced in master too, so I don't think it's related to this PR. make release works fine however.

[zorgiepoo]

Ah, I think it's the target dependency ordering. Place generate_appcast after the other Sparkle targets for the Distribution and All targets; I trust you can make that change @pornel

[kornelski]

I was unable to reproduce the build failure. All and Distribution worked fine for me.

[zorgiepoo]

The errors I am running into is symbols like SUError and SUStandardVersionComparator being unresolved (Swift code not finding those in the obj-c bridging).

One workaround for me I found I believe is adding Sparkle.framework as a target dependency to generate_appcast, and adding 'import Sparkle' in Appcast.swift and DSASignature.swift, to get both make release to work for me and building under Distribution target in Xcode. But this is kind of strange.. @pornel

[kornelski]

I've seen that briefly, but I think it's an Xcode bug, because it goes away after Clean Build Folder.

[zorgiepoo]

Yeah, I think you're right..