Fix multi-line comment warnings

[paleolimbot]

From CRAN after #212.

Thanks, reverse dependency checks have been triggered, but there are a few hundred warnings about


../src/absl/base/config.h:341:1: warning: multi-line comment [-Wcomment]
  341 | // #if (defined(__clang__) && !defined(_WIN32)) || \
      | ^

../src/absl/base/internal/raw_logging.h:47:3: warning: multi-line comment [-Wcomment]
   47 |   // do {                   \
      |   ^

which should be fixed for future submissions.