allow on stack with betterC

[Dennis]

On Tuesday, 23 October 2018 at 10:18:56 UTC, test wrote:
> I can not user static array because the length is a runtime 
> vars.

Then you can either:
- use malloc anyway (recommended when the array can be quite 
large)
- allocate a static array with an upper bound, and only use a 
slice of it
- use alloca to allocate on the stack. Watch out for stack 
overflows though.
```
import core.stdc.stdlib: alloca;
import core.stdc.stdio: printf;
extern(C) void main()
{
     int n = 5;
     ubyte[] tmp = (cast(ubyte*) alloca(ubyte.sizeof * n))[0..n];
     tmp[4] = 22;
     printf("%d", tmp[4]);
}
```
When compiling with dmd and -betterC I got a linker error 
"undefined reference to '__alloca'", but with LDC it worked fine.