Make --disable-ipv6 the default?

[brettcannon]

Or is the hope that Emscripten may support it someday, and so making it an opt-out still makes sense?

[emmatyping]

We should probably look into where things break and ask if upstream has plans to support it (they may already, I'm not certain). If we can enable it, it would be good to do so.

[tiran]

I wonder why the flag is necessary. Is this perhaps related to buggy_getaddrinfo check in configure? The check uses AC_RUN_IFELSE, which obviously does not work for cross compiling. The ac_cv_buggy_getaddrinfo=no` override` in CONFIG.SITE`` should be good enough to work around the problem.