ALLOW_UNIMPLEMENTED_SYSCALLS doesn't exclude unsupported syscalls in autoconf

A program I was cross-compiling relies on `pipe2`. I've noticed that it was always printing a warning about `pipe2` being unsupported during runtime, and always falling back to `pipe`.

While it's good that they left in a runtime fallback, I was wondering if there's a way to avoid compiling that unsupported code in altogether. I've noticed that they detect `pipe2` with `configure.ac`:

```
AC_CHECK_FUNCS([pipe2])
```

When executed with Emscripten, it prints:

```
checking for pipe2... yes
```

After looking through settings, I've assumed it's because `ALLOW_UNIMPLEMENTED_SYSCALLS` is enabled by default, so I went ahead and changed it, but, surprisingly, now `configure` continued to report

```
checking for pipe2... yes
```

but the actual program fails to link due to the missing syscall.

Looking at `config.log`, this is the only relevant bit which seems suspicious but I don't understand how it explains why `configure` still manages to find the `pipe2` function:

```
configure:17216: checking for pipe2
configure:17216: /usr/local/google/home/rreverser/emsdk/upstream/emscripten/emcc -o conftest -g -O2 -s ALLOW_UNIMPLEMENTED_SYSCALLS=0 conftest.c >&5
wasm-ld: warning: function signature mismatch: pipe2
>>> defined as () -> i32 in /tmp/emscripten_temp_nj0l58ag/conftest_0.o
>>> defined as (i32, i32) -> i32 in /usr/local/google/home/rreverser/emsdk/upstream/emscripten/cache/sysroot/lib/wasm32-emscripten/libc.a(pipe2.o)
configure:17216: $? = 0
configure:17216: result: yes
```

Does it try to link against wrong `pipe2` function or something?