Chose opentxs library path, fix rpm

Now you should be able to chose the path where your opentxs is with
-DCMAKE_PREFIX_PATH=/home/lucas/tmp/opentxs/


Can somebody test that on his system?

To install opentxs in another directory than the system default (/usr/lib) do
cmake .. -DCMAKE_INSTALL_PREFIX=/home/your/prefix/