Fix clean target of C++ projects

When migrating to git we deleted the delnode.exe tool which broke
the clean target of the nmake files. Fortunately rmdir has
learned to recursively delete directories so that we don't need
a special tool anymore.

Change-Id: Ie7e50a8a42b4fdec559b21c8dc3ea5f52f987822
Fix resource leak

MSDN documentation says that we should call ImmReleaseContext
when we're done with the context.

Change-Id: I0a5065fe546814945d0acd1dfe1d67d3e0f2ccd4