These OS's don't always have HOST_NAME_MAX defined, resulting in
build errors.

A few related changes as well:
* Only define HOST_NAME_MAX if it's not already defined. There are
  some cases where this is already defined, e.g. with NaCl if
  __USE_POSIX is set. To avoid all of these, only define it if it's
  not already defined.
* Default HOST_NAME_MAX to 64 and issue a #warning. Having the wrong
  max length is pretty harmless. The name just ends up getting
  truncated and this is only for printing debug info. Because we're
  constructing a std::string from a char[] (so defined length), we
  don't need to worry about gethostname's undefined behavior for
  whether the truncation is null-terminated when the hostname
  doesn't fit in HOST_NAME_MAX. Of course, this doesn't help people
  who have -Werror set, since they'll still get a warning.