Since we require C11 now, we can assume many headers exist, and
clean up all of the conditional includes. It's not like any of
this code actually accounted for the headers not existing, just
whether we could include them.
The strings.h cleanup is a little nuanced: it isn't in C11, but
every use of it in the codebase will include strings.h only if
string.h doesn't exist. Since we now assume the C11 string.h
exists, we'll never include strings.h, so we can delete it.
