[v3] concept check macros, excess-errors

[Benjamin Kosnik]

> Thus libstdc++.so has a boatload of undefined references to various
> instantiations of that function.  (At one point I thought it wasn't actually
> being called, but that was for the old implementation.)
> 
> So, "hello world" fails to link because
> __glibcpp_function_requires<YourFavoriteConcept>() isn't found, etc.

can't you just add them as explicit instantiations in src/stl-inst.cc?