Variable-Length Arrays in C89 – Compatibility and Alternatives

I've read that C89 does not support variable-length arrays, but the following experiment seems to disprove that:

#include <stdio.h>

int main()
{
   int x;
   printf("Enter a number: ");
   scanf("%d", &x);
   int a[x];
   a[0] = 1;
   // ...
   return 0;
}

When I compile as such (assuming filename is va_test.c):

gcc va_test.c -std=c89 -o va_test

It works…

What am I missing? 🙂