Issue: headers may use non-reserved identifiers

Hi all,

Issue: headers may use non-reserved identifiers.

Example:
#define _reent 0
#include <stdio.h>

$ gcc t567.c -std=c11
t567.c:1:16: error: expected ‘{’ before numeric constant
    1 | #define _reent 0
      |                ^
t567.c:1:16: error: expected ‘{’ before numeric constant
    1 | #define _reent 0
      |                ^
and so on...

Per C11 _reent, _on_exit_args, etc. are non-reserved identifiers.

Consider fixing.

P.S. Good if it leads to compile time errors, not good if it doesn't (wrong
translation unit produced => wrong code generated => wrong runtime
behavior).

--
Pavel