Right now Doxygen is documenting everything in the repo including submodules, everything in contrib,
util, tests, etc. This bogs down the documentation to the point where it is very hard to navigate. I think
it would be a good move to focus on documenting only the main C++ code which is specific to this repo.
Right now this means documenting `src/` (without SUPERCOP), `contrib/epee/`, `external/easylogging++`. After this commit,
Doxygen went from running >6000 graphs to about 2200 graphs.
Changes the Doxyfile to expand preprocessor macros, but only the ones
defined in the Doxyfile. This way we can specify that
BLOCKCHAIN_DB == DB_LMDB for the sake of documentation.