Makefile: Fixed the CryptoPP build by copying the directory into the right place

It was failing before because something created that directory beforehand
and the lib/CryptoPP was copied into build/CryptoPP/CryptoPP.
This change should make it more robust against these types of
subtle failures.
Merge pull request #23 from muelli/make_cp

Makefile: Fixed the CryptoPP build by copying the directory into the rig...