Merge r233749, building libsupc++ as a shared library and making libstdc++ a
filter library.

It should now be possible to build applications on 9-STABLE that link against
both libc++ and libstdc++ and to test libcxxrt with libstdc++.

If you wish to test libcxxrt, please add this to your /etc/libmap.conf:

libsupc++.so.1 libcxxrt.so.1

If you wish to test libc++, add -std=libc++ to your compile and link flags for
your favourite C++ applications and / or libraries.