Function that are undefined in c/c+, because defined c/c++ in debug version.
I currently noticed that if you download the c/c++ glad file for 4.5 core and use it in a system supports less then 4.5 then in the release build all function pointers that are not available with the version before 4.5 point to _nothing_, so you could write something like that:

```
if( !glCopyNamedBufferSubData ) {
     // do some fallback if glCopyNamedBufferSubData does not exists
}
```

But in the debug build all functions have the `_pre_call_callback`, `_post_call_callback`  wrapper, so all functions are set even if they are not support by the current platform.

Is the behaviour intended?
