Alloca without core.stdc.stdlib?

[ARaspiK via D.gnu]

According to core.stdc.stdlib, alloca (on GDC) is a compiler 
intrinsic.
But I can't separate it from the rest of core.stdc.stdlib, for a 
small druntime I'm making. Here's what it seems to be:

version(GNU) extern(C) @system nothrow @nogc void* alloca(size_t 
size) pure;

Writing the declaration to a file, or even removing everything 
else from core.stdc.stdio, isn't working. I cannot have any C 
library stuff. How do I isolate it?