makefile.unix creates obj directory and depends on and builds cryptopp

This fixes two issues with building the maxcoind on linux.

1. Creates the src/obj directory, needed to compile the source.
2. Actually builds libcryptopp and links with it, instead of just trying to link with cryptopp/libcryptopp.a (that doesn't magically exist before it's built).

compile as per the instructions:
cd src && make -f makefile.unix