Add iOS CMake support

[crazyhappygame]

In CMake https://github.com/firebase/firebase-cpp-sdk/blob/master/release_build_files/CMakeLists.txt#L31 there is only :

elseif(APPLE)
    set(FIREBASE_SDK_LIBDIR ${FIREBASE_CPP_SDK_DIR}/libs/darwin/universal))

Please add also ios branch

[crazyhappygame]

FYI. Starting from cmake 3.14.0 there is native support for iOS:
https://cmake.org/cmake/help/latest/manual/cmake-toolchains.7.html#cross-compiling-for-ios-tvos-or-watchos

Some game engines like cocos2d-x https://github.com/cocos2d/cocos2d-x switched to cmake

[stewartmiles]

We've supported this since we first open sourced the project. See https://github.com/firebase/firebase-cpp-sdk#building-with-cmake-for-ios

[crazyhappygame]

This ticket is not about building but about using in iOS cmake project

[crazyhappygame]

@stewartmiles please reopen this issue

[stewartmiles]

@crazyhappygame it looks like you opened the feature request #22 feel free to follow up on there.

[crazyhappygame]

@stewartmiles there are 2 things:

1. Building firebase-cpp-sdk with CMake
2. Using firebase-cpp-sdk in CMake project

There are both about CMake but those are completely different use cases....