message: compiler: Add unused result warning attribute.
diff: # define ELLE_COMPILER_ATTRIBUTE_NORETURN __declspec(noreturn)
  # define ELLE_COMPILER_ATTRIBUTE_NO_INLINE __declspec(noinline)
  # define ELLE_COMPILER_ATTRIBUTE_PURE
  +# define ELLE_COMPILER_ATTRIBUTE_WARN_UNUSED_RESULT _Check_return_
  # define ELLE_COMPILER_PRETTY_FUNCTION __FUNCSIG__
  # define constexpr
  #else // Gcc and clang.
  # define ELLE_COMPILER_ATTRIBUTE_NORETURN __attribute__((noreturn))
  # define ELLE_COMPILER_ATTRIBUTE_NO_INLINE __attribute__((noinline))
  # define ELLE_COMPILER_ATTRIBUTE_PURE __attribute__((pure))
  +# define ELLE_COMPILER_ATTRIBUTE_WARN_UNUSED_RESULT \
  + __attribute__((warn_unused_result))
  # define ELLE_COMPILER_PRETTY_FUNCTION __PRETTY_FUNCTION__
  #endif
  