Feature test macro review (cont.)

Feature test macro review (cont.)

I found a couple of symbols I missed during the previous macro review:

- getline(). This forces POSIX >= 2008.
- IN6_ARE_ADDR_EQUAL(). I can't find where this is supposed to be
  standardized, so I decided to ditch it.

Also, declaring these macros in every dependent file is scaling poorly,
so I moved them to the CC directives, unifying them in the process.
_POSIX_C_SOURCE is now 200809L, and _XOPEN_SOURCE is 700 (to match).

Also, reduce -std to c99. I don't really have an issue with gnu11, but
it looks like the delta is heavily underutilized in the project.
Might revert it later.

Finally, update the unit tests.