GL_STACK_OVERFLOW and GL_STACK_UNDERFLOW seem to be undefined
I've just generated an OpenGL 2.1 glad zip from the web service (I'm trying to switch a quite large project away from GLEW) and it appears GL_STACK_OVERFLOW and GL_STACK_UNDERFLOW are undefined. In fact, this is the section in glad.h where they should be...

```
#define GL_NO_ERROR 0
#define GL_INVALID_ENUM 0x0500
#define GL_INVALID_VALUE 0x0501
#define GL_INVALID_OPERATION 0x0502
#define GL_OUT_OF_MEMORY 0x0505
```

Notice that it jumps from 0x0502 to 0x0505 :)
