[PATCH] Use __builtin_bswap32/64 in __bswap_32/64 for GCC >= 4.2

[Roland McGrath]

Please clean the file up to use __GNUC_PREREQ everywhere.
That's much easier to follow.

You're not just changing the behavior for GCC >= 4.2, which is what the log
entry says.  You're also dropping the existing code for 2.0 <= GCC < 4.2.
That seems questionable.  Please explain what effects this has on user code
compiled with e.g. GCC 4.1.


Thanks,
Roland