Use a heavier hammer when --enable-libcpp is passed to bypass the tests
which catch buggy versions of libstdc++. While libc++ would pass them,
we don't actually update the state in the configure script to use libc++
when we pass --enable-libcpp, the logic for that is in the
Makefiles. So just completely skip the library test when that configure
flag is passed.

Hopefully this will be enough to fix the darwin bots at last, and thanks
to Duncan Smith for getting things set up so I can watch the bots myself
on lab.llvm.org and see any failures!