Disable standalone executable support under WASM / Emscripten

[saghul]

Closes: #1527

[bnoordhuis]

Instead of commenting out big blocks of code, it's more robust to do something like:

#if defined(EMSCRIPTEN) || defined(__wasi__)
#define emscripten_or_wasi 1
#else
#define emscripten_or_wasi 0
#endif

And then instead of ifdef'ing out code, like this:

#if !defined(EMSCRIPTEN) && !defined(__wasi__)
do_the_thing();
#endif

You do:

if (!emscripten_or_wasi) {
    do_the_thing();
}

That way all code is always compiler-checked.

[bnoordhuis]

Maybe this instead?

Suggested change

	} else if (expr) {
	#else
	if (expr) {
	#endif
	}
	#endif
	if (!standalone && expr) {

Arguably mildly easier to read.