message: Fix formatting and newline-escaping
diff: # define CAF_UNLIKELY(x) __builtin_expect((x), 0)
  # define CAF_DEPRECATED __attribute__((deprecated))
  # define CAF_DEPRECATED_MSG(msg) __attribute__((deprecated(msg)))
  -# define CAF_PUSH_WARNINGS
  +# define CAF_PUSH_WARNINGS \
  _Pragma("GCC diagnostic push") \
  _Pragma("GCC diagnostic ignored \"-Wshadow\"") \
  _Pragma("GCC diagnostic ignored \"-Wpragmas\"") \
  # define CAF_POSIX
  #endif
  -#if defined(CAF_WINDOWS)
  -# if defined(__clang__)
  - struct IUnknown; // fix for issue with static_cast<> in objbase.h (see https://github.com/philsquared/Catch/issues/690)
  -# endif
  +#if defined(CAF_WINDOWS) && defined(CAF_CLANG)
  +// Fix for issue with static_cast<> in objbase.h.
  +// See: https://github.com/philsquared/Catch/issues/690.
  +struct IUnknown;
  #endif
  #include <cstdio>
  