Fix Tru64 problem with stdbool.h. * lib/stdbool.in.h (false, true): [! (defined __cplusplus || defined __BEOS__) && !defined __GNUC__]: Don't declare as an enum in this situation; it runs afoul of Tru64. Problem reported by Steven M. Schweda in <http://lists.gnu.org/archive/html/bug-autoconf/2007-10/msg00019.html>.