Use GCC's -Wsuggest-attribute warning

[MisterDA]

The GCC -Wsuggest-attribute=attribute_name warning warns for cases where adding an attribute may be beneficial. I've used it for the format, noreturn, malloc, and returns_nonnull.
I've also tried with pure and const but found out that these mostly apply to primitives, thus adding the attribute brings no benefit.

• About format:
  The GCC warning -Wsuggest-attribute=attribute_name can be used to suggest to apply attributes to functions. Enable it to find all candidates for the format attribute, and apply it.

  A few notes:

  • The wprintf archetype for the format attribute is not supported;
  • attributes are applied to function names, not types, thus it is not possible to apply the format attribute inside the Logger typedef, thus the macro. As this change is constrained to ocamltest, I believe it is not too invasive.

• About noreturn:
  Use GCC -Wsuggest-attribute=noreturn warning to find functions possibly missing a noreturn attribute. Applying the noreturn attribute allows removing the placeholder returns.

  The GCC manual has this line about the noreturn attribute:

  It does not make sense for a noreturn function to have a return type other than void.

  Change the return type of some primitives when they always throw an exception.

• About surround attribute names with __ in headers:
  The GCC docs about attribute syntax suggests this:

  You may optionally specify attribute names with __ preceding and following the name. This allows you to use them in header files without being concerned about a possible macro of the same name. For example, you may use the attribute name __noreturn__ instead of noreturn.