Move the C++ api files under a new subdirectory.
Use CMAKE_DEPENDENT_OPTION to enforce sdt->cpp_api relationship.

Since linking .a into a .so can cause global symbols to be dropped, add
a helper file to force exported symbols to be retained (link_all.cc).
This problem doesn't exist for building the static cpp examples.
Signed-off-by: Brenden Blanco <bblanco@gmail.com>