message: elle.print: avoid GCC warnings
diff: @@ -50,6 +50,18 @@ namespace elle
  | Helpers |
  `--------*/
  + // GCC 4.9, when instantiating this piece of code, generates tons
  + // of warnings about our `-> decltype(bool(t))`:
  + //
  + // print.hxx:57:19: warning: the compiler can assume that the address
  + // of 't' will always evaluate to 'true' [-Waddress]
  + //
  + // The pragma cannot be restristed to that line.
  +#if defined __GNUC__ && ! defined __clang__
  +# pragma GCC diagnostic push
  +# pragma GCC diagnostic ignored "-Waddress"
  +#endif
  +
  /// Boolean value of types featuring to bool() operator.
  template <typename T>
  auto
  @@ -59,6 +71,10 @@ namespace elle
  return bool(t);
  }
  +#if defined __GNUC__ && ! defined __clang__
  +# pragma GCC diagnostic pop
  +#endif
  +
  /// A string is "false" iff it's empty.
  ///
  /// We also support dubious derived classes. See das's
  