Update build instructions for Berkeley DB

- People were having problems with the .so when installing in alternative locations.
  Like gitian, build a static library with -fPIC that can be embedded into the executables

- Add some missing steps, and mention how to use our self-built version of BDB in the bitcoin build

- Add reminder that BerkeleyDB is only needed when wallet support is enabled

- Log BerkeleyDB version at startup (if wallet support enabled)

https://github.com/laanwj/bitcoin/blob/2014_05_berkeleydb_version/doc/build-unix.md