SPM build warnings

[pepone]

We get this warnigs when building for macOS

clang: warning: -framework ExternalAccessory: 'linker' input unused [-Wunused-command-line-argument]

probably because iAP sources are wrapped with #if defined(__APPLE__) && TARGET_OS_IPHONE != 0 gitve this farmework is supported with macOS we can consider build iAP with macOS too

I don't see a way to support different linker settings with iOS / macOS with SPM