The following program produces compile time warnings from gcc:

namespace Test
{
	public static int main (string[] args)
	{
		unowned string[] dirs = Environment.get_system_data_dirs();
		
		return 0;
	}
}

Gives:

  foo.vala.c: In function ‘test_main’:
  foo.vala.c:25: warning: assignment from incompatible pointer type

And indeed on line 25 of the generated C it assigns a const char** to a char**.

The VAPI appears to be correct as it declares this function as

  public static unowned string[] get_system_data_dirs ();