libhdfs++: Add a build option to skip building examples, tests, and tools

Adding a flag to just build the core library without tools, examples, and tests will make it easier and lighter weight to embed the libhdfs++ source as a third-party component of other projects. It won't need to look for a JDK, valgrind, and gmock and won't generate a handful of binaries that might not be relevant to other projects during normal use.

This should also make it a bit easier to wire into other build frameworks since there won't be standalone binaries that need the path to other libraries like protobuf while the library builds. They just need to be around while the project embedding libhdfs++ gets linked.