cpplib.h (struct cpp_options): Add new boolean flag warn_builtin_macro_redefined.
* include/cpplib.h (struct cpp_options): Add new boolean flag
warn_builtin_macro_redefined.
* init.c (cpp_create_reader): Initialize warn_builtin_macro_redefined.
* (struct builtin_operator): Split out from previous struct builtin,
enhance extra const correctness.
* (struct builtin_macro): Split out from previous struct builtin, add
new always_warn_if_redefined flag, enhance const correctness.
* (mark_named_operators): Use struct builtin_operator.
* (cpp_init_special_builtins): Use struct builtin_macro, add NODE_WARN
to builtins selectively.
* macro.c (warn_of_redefinition): Return false if a builtin macro
is not flagged with NODE_WARN.