slow V3 configures (was Re: ICE during bootstrap.)

[Joe Buck]

> we check for c++ declarations and c linkage. both are necessary. If you 
> can figure out a faster way to do it, I'm all ears.

Suppose you run one test that passes only if all the "common" functions
are present and declared correctly, and if this test fails, then you run
individual tests?  What remains is to decide what is "common".  One
possible goal is to choose a set that will pass in one shot on most
modern platforms -- but we still have a fallback on the other platforms.