[PATCH] byteswap-16.h: USe __builtin_bswap16 in gcc 4.8

[Roland McGrath]

We won't include such a change until 4.8 is actually released (or very
close ot it).  Before that, there is no real guarantee what things are
or aren't assumable from __GNUC_PREREQ (4, 8).

Regardless, please read the wiki contribution checklist and follow up
with those details: proper log entry, posted in correct format;
report on testing done; as a new contributor, verify the status of your
copyright papers.


Thanks,
Roland