My Firebase iOS App i had to put the frameworks in manually due to some reason in cocoapods.
i have been getting an crash error saying
2016-06-04 00
I am using Xcode 7.3.1 targeting iOS 9.3 on OS X 10.11.5.
I had the exact same issue as above with the "Undefined symbols" issue.
I first emptied everything from the "Other Linker Flags" and then added $(OTHER_LDFLAGS) and -ObjC as 2 separate items (using the + button when you double click).
The other thing I did was remove the frameworks giving me issues - GoogleUtilities and FirebaseAnalytics.
I then added them back in using the "Add files to Project" option one at a time and ensured they are targeted to my iOS app.
The build was successful after that and ads show up.