message: print: Fix test on GCC 4.9.
diff: @@ -150,8 +150,7 @@ conditional_positional()
  BOOST_TEST(print("{0?({0})}", "foo") == "(foo)");
  BOOST_TEST(print("{0?({0})}", "") == "");
  - BOOST_TEST(print("{0?({0})} - {1?({1})} - {2?({2})}",
  - 0, 1, "2"s)
  + BOOST_TEST(print("{0?({0})} - {1?({1})} - {2?({2})}", 0, 1, std::string("2"))
  == " - (1) - (2)");
  }
  