As per [lex.name],

Each identifier that contains a double underscore __ or begins with an underscore followed by an uppercase letter, other than those specified in this document (for example, __cplusplus ([cpp.predefined])), is reserved to the implementation for any use.
(3.2)

Each identifier that begins with an underscore is reserved to the implementation for use as a name in the global namespace.

Macros for internal usage are not supposed to begin with an underscore. Updated the naming of the internal macros to follow the standard naming conventions. No functional changes.