Inproper define "#if INTERNAL_EXPERIMENTAL" should be changed to "#ifdef INTERNAL_EXPERIMENTAL"

[crazyhappygame]

Please fill in the following fields:

Pre-built SDK from the website or open-source from this repo: prebuild
Firebase C++ SDK version: firebase_cpp_sdk_6.12.0
Firebase plugins in use (Auth, Database, etc.): Auth
Additional SDKs you are using (Facebook, AdMob, etc.): None
Platform you are using the C++ SDK on (Mac, Windows, or Linux): Any
Platform you are targeting (iOS, Android, and/or desktop): Any

Please describe the issue here:

MSVC compile with: /W4
compilation warning

firebase_cpp_sdk\include\firebase/app.h(232,5): warning C4668: 'INTERNAL_EXPERIMENTAL' is not defined as a preprocessor macro, replacing with '0' for '#if/#elif'

This is regression compare to 6.2.0
Problem:
Below line

#if INTERNAL_EXPERIMENTAL

Should be replaced with

#ifdef INTERNAL_EXPERIMENTAL

Please answer the following, if applicable:

Have you been able to reproduce this issue with just the Firebase C++ quickstarts ? yes

What's the issue repro rate? 100%

[a-maurice]

Thanks for alerting us of this, we will push a fix for this as soon as we can.

[chkuang-g]

@crazyhappygame

The patch should be in master branch now