[cartertemm]

Static builds on macOS and iOS fail to link, and when I hacked them into working, no backends are registered. This change fixes both problems.

• build.rs only links c++ for static Apple builds. prism also needs system frameworks, so a minimal macOS binary fails with undefined IOKit and CoreFoundation symbols from the power notifier.
• The pinned prism commit doesn't keep backend objects in static builds made with GCC or Clang, which is why backend_count() returns 0 and create_best() fails. Looks like you fixed this upstream.

I bumped the prism submodule to 6d87910, which includes the upstream fix
for static backend registration (ethindp/prism#135). I also link the frameworks prism uses, based on PrismPlatformApple.cmake:

• macOS: Foundation, AVFoundation, AppKit, IOKit, and CoreFoundation.
• iOS: Foundation, AVFoundation, and UIKit.

This also necessitated a README note for Xcode projects that link a Rust staticlib. Cargo doesn't do the final link in that case, so those projects must add the
frameworks themselves.

Testing

I built a binary with
prismer = { default-features = false, features = ["static"] } and no extra link flags.

• macOS arm64: builds, and 2 backends register.
• iOS simulator (arm64): builds, and 2 backends register.
• Default shared build: the speak example still works.
• Clippy passes for the default and static builds.

Not covered

• Linux static builds still don't link, because build.rs doesn't link the
  pkg-config libraries for Orca, speech-dispatcher, and power management. If you like, I can open a separate PR for that.
• tvOS, watchOS, and visionOS aren't handled at all. I haven't built apps for these yet so am uncertain as to what we'd need to do there.

[cartertemm]

Just noticed #3, which handles the linux case as well.

I think this PR can be closed accordingly, however, the note in the readme should be added there too.

[trypsynth]

Thanks for this, and for testing it on macOS and the iOS simulator! Main already covers the same ground: 3ff0dbe links the Apple frameworks for static builds (on tvOS, visionOS and watchOS too, plus libobjc), and the prism submodule on main is past ethindp/prism#135, so backends register. Your README note about Xcode projects linking a Rust staticlib wasn't there, though, so I've added it to main in 430afe9, with you as co-author. Closing this, since the rest is already in.