[MERGE] Branch prediction hints

Hi all,
        This patch supersedes yesterday's; it impements autoconf-based
detection of compiler builtins.

The purpose of this feature is to give explicit hints to compilers
supporting them about the likely outcome of an if() clause; thus
allowing the compiler to do explicit optimizations (CPU predictor
hinting, code layout optimizations) based on the predicted outcome.
The obvious case is the singleton pattern, where the existence check
will always be true except for the first time.

This work is not correlated to Alex's SMP-inspired cachemgr work.

        Kinkie