nested autoconf package rather than as raw source.  This way we can
trivially update it again in the future.

Actually, this change doesn't even include gtest in protobuf's SVN.
Instead, we auto-download it when autogen.sh is invoked.  Note that
it will be included in release distributions, though.

TODO:
* Add a configure option to use the system's installed gtest rather
  than the bundled copy.  Apparently the gtest maintainers are working
  on some general-purpose autoconf macros which will do this
  automagically.
* Update MSVC project files.