Update C++ Makefile

Make minor various updates to the new C++ Makefile added by previous
commit. Specifically:
 * Add comments throughout
 * Add doc, examples and help targets
 * Moved all examples to a dedicated dir to make them easier to find
 * Changed dist target to build packages in dist directory
 * Add clang installation in core ubuntu_install.sh package to make
   it easier for user to have clang installed. Also add g++ fallback
   if clang is not available
 * Changes to .gitignore, main Makefile and C++ README to reflect
   changes