firebase-ios-sdk

Add a cc_library to the CMake build (#670)

* Rewrite cc_test to take named arguments

Cut down on build file verbosity by having cc_test take SOURCES and
DEPENDS. The separate invocation of target_link_libraries is no longer
necessary.

* Add a cc_library rule to parallel cc_test

This cuts down on build file verbosity.

* Automatically add OBJC_FLAGS to cc_libraries if applicable

* Exclude platform-specific libraries from 'all'

This is makes it possible to declare this kind of library
unconditionally. Usage within a test or as a dependency will actually
trigger building.

* Restore secure_random_test.cc; clean-up comments