This allows running the test suite against the native Standard Library
on most systems, and against libstdc++ installed at a custom location.
Of course, these configurations don't run 100% clean at the moment. In
particular, running against the native stdlib is almost guaranteed not
to work out-of-the-box, since the test suite generally contains tests
for things that have been implemented on tip-of-trunk but not released
to most major platforms yet. However, having an easy way to run the test
suite against that library is still both useful and interesting.
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