VMS Help CC, Messages, BOOLNA  *Conan The Librarian (sorry for the slow response - running on an old VAX)

 Message        The _Bool keyword is not supported in this language
                mode.  It will be treated as an identifier in this
                compilation.

 Description    Support for the _Bool keyword is only available in
                certain language modes.  Support is not present when
                the compiler is in VAX C, K & R (common), or strict
                ANSI89 standard modes.  In these language modes _Bool
                will be treated as an identifier.

 User Action    Compile using one of the other compilation modes.