Build and test both C++ loggers where possible (#595)

* Rename FIREBASE_BINARY_DIR to FIREBASE_INSTALL_DIR

  Make this consistent with the outer superbuild and also make the
  association with CMAKE_INSTALL_PREFIX more obvious.

* Build and test log_stdio separate from the rest of util

  This is in preparation for adding a test for log_apple

* Build and test log_apple under CMake

  Also add notes about how FIRLogger's debug mode can break this test

* Refactor log_apple to cut down duplicate switch statements

  There's also a slight reduction in final binary size.

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