firebase-ios-sdk

Rework config.h handling for Swift Package Manager compatibility (#6087)

Swift Package Manager does not support configure-time feature testing
and does not support source transformations of any kind so we can't
cheat by running a sed command to generate config.h before building.

This change makes it so that CMake now generates a config_detected.h
while config.h always exists and contains fallback values if the build
system cannot generate config_detected.h.

Convert CocoaPods to use the same fallbacks since the values are the
same as SPM.