Fix some build issues with the "core" component selection (rdar://47394086)

Fix some build issues with the "core" component selection (rdar://47394086)

- Availability macros were incorrect.
- Some driver headers/definitions were installed even when the core library
  does not contain them.
- Fix up the macOS/iOS detection of keychain/API availability - just need to
  use the TARGET_OS_xxx macros instead.
- When installing private headers, install config.h so that the private
  headers actually work.