[v3] revamp header pedantic testing

[Steve Ellcey]

>                                     What is wierd is that if I compile
> all_pedantic_errors.cc (which has nothing but '#include <bits/extc++.h>'
> in it) using the just built compiler from its installed location, it
> works fine. But it I compile it the way the test run does (with the -B's
> and -I's, out of the build directory), then it fails.  I am trying to
> figure out where the two compilations diverge but I was wondering if you
> had any thoughts on this problem.
> 
> Steve Ellcey
> sje@cup.hp.com

Following up on my own email, it looks like the reason I get this error
when running the testsuite from the build location and not from the
installed location is that we use -I to include the new C++ header files
from the building locaion intead of -isystem.  -isystem would suppress
these messages (even if -pedantic-errors is used) and allow the tests to
pass.  I am going to test a change to libstdc++-v3/acinclude.m4 to use
-isystem instead of -I.

Steve Ellcey
sje@cup.hp.com