[wasm2c] alloca.h about wabt

[keithw]

The best way to fix this is probably just to edit src/template/wasm2c.includes.c. Right now it says:

#if defined(_MSC_VER)
#include <intrin.h>
#include <malloc.h>
#define alloca _alloca
#else
#include <alloca.h>
#endif

If there's a different header that FreeBSD wants, seems easy to detect and include it here. We probably shouldn't do this in CMake because (at least so far) wasm2c generates the same portable output on any host platform.

[penzn]

FreeBSD way of doing it would be to include stdlib.h. I can put that under an ifdef or just replace alloca.h with it as it would work on other posix systems as well.

[keithw]

I think I'd prefer an ifdef for FreeBSD -- Linux's documentation calls for alloca.h, and stdlib is more to pull in if not required.