Update README.cmake regarding Unix, C++ and tests

Update README.cmake regarding Unix, C++ and tests

Issue #105 (bdwgc).

* doc/README.cmake: Mention that Unix targets are also supported;
remove not that support is not yet complete; update the list of
supported targets (that CMake is able to generate for); use Visual
Studio 9 instead of Visual Studio 8 in the examples; document
enable_cplusplus and build_tests options; provide an example with
--config option; change "build" artifacts folder to "out" one in
the example.