firebase-ios-sdk

CMake fixes (#2966)

* cc_binaries should not be tests

This will keep more involved benchmarks from extending test time
unreasonably.

* Fix target_link_libraries conformance with CMP0023

Avoid mixing keyword and non-keyword forms of target_link_libraries.

* Set target_include_directories to PRIVATE for cc_binary and cc_test

* Set add_sources to private

Fixes a warning about policy CMP0076, which wants to make sources
relative if they're public.

* Enable @rpath by default

This silences warnings about policy CMP0042, which we don't care much
about since we don't have install targets.

* Disable griping about undeclared byproducts

... when using the Ninja generator. In our case the undeclared outputs
come from dependencies and we'd need to fix them before we can adopt the
new policy here.