Do not define unsupported IPPROTO_SCTP

[emmatyping]

Hi! In working on CPython porting to Emscripten I discovered that IPPROTO_SCTP seems to be defined but when used leads to a failed assert.

My emcc -v output:

emcc (Emscripten gcc/clang-like replacement + linker emulating GNU ld) 3.0.0 (3fd52e107187b8a169bb04a02b9f982c8a075205)
clang version 14.0.0 (https://github.com/llvm/llvm-project 4348cd42c385e71b63e5da7e492172cff6a79d7b)
Target: wasm32-unknown-emscripten
Thread model: posix
InstalledDir: /home/ethanhs/emsdk/upstream/bin

I'm afraid I don't have a small easy reproduction, but you can reporduce the assertion by cloning https://github.com/ethanhs/python-wasm and pull my PR python/cpython#30495 to the cpython sub-directory. Here is the backtrace I get:

Aborted(Assertion failed: undefined)
exiting due to exception: RuntimeError: Aborted(Assertion failed: undefined),RuntimeError: Aborted(Assertion failed: undefined)
    at abort (/home/ethanhs/python-wasm/cpython/builddir/host/python.js:1511:11)
    at assert (/home/ethanhs/python-wasm/cpython/builddir/host/python.js:697:5)
    at Object.createSocket (/home/ethanhs/python-wasm/cpython/builddir/host/python.js:4719:11)
    at ___syscall_socket (/home/ethanhs/python-wasm/cpython/builddir/host/python.js:6643:25)
    at socket (<anonymous>:wasm-function[8038]:0x2e72af)
    at sock_initobj (<anonymous>:wasm-function[5651]:0x23728c)
    at wrap_init (<anonymous>:wasm-function[3135]:0x140540)
    at wrapperdescr_call (<anonymous>:wasm-function[468]:0x20c48)
    at _PyObject_MakeTpCall (<anonymous>:wasm-function[2352]:0xfda86)
    at PyObject_Vectorcall (<anonymous>:wasm-function[2360]:0xfe197)

which is the assert

var streaming = type == 1;
if (protocol) {
  assert(streaming == (protocol == 6)); // if SOCK_STREAM, must be tcp
}

[sbc100]

I'm pretty sure thee is no support for SCTP in emscripten... even TCP isn't really supported, except via some kind of proxy server. Can you force the configure process for python to report these features as missing?

[emmatyping]

@tiran was the one who initially found this issue, and suggested I open an issue.

Right now we are just checking for __EMSCRIPTEN__, but ideally the value would not be defined if it is not supported I think.

[sbc100]

IPPROTO_SCTP is a macro defined in the musl headers. We don't normally go through and prune the musl headers for such things.

Also, I', not sure the existence of the IPPROTO_SCTP in the libc headers says anything about whether it will work at runtime does not? ... it just allows programs that contain this macro to compile.

Do you know what code its trying to use IPPROTO_SCTP? Perhaps we should just return an error instead of asserting? Would that make the code happy?