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

- Add reminder that BerkeleyDB is only needed when wallet support is
  enabled

doc/build-unix.md

@@ -137,17 +137,33 @@ miniupnpc
 
 Berkeley DB
 -----------
-You need Berkeley DB 4.8.  If you have to build it yourself:
+It is recommended to use Berkeley DB 4.8. If you have to build it yourself:
 
+```bash
+BITCOIN_ROOT=$(pwd)
+
+# Pick some path to install BDB to, here we create a directory within the bitcoin directory
+BDB_PREFIX="${BITCOIN_ROOT}/db4"
+mkdir -p $BDB_PREFIX
+
+# Fetch the source and verify that it is not tampered with
 wget 'http://download.oracle.com/berkeley-db/db-4.8.30.NC.tar.gz'
 echo '12edc0df75bf9abd7f82f821795bcee50f42cb2e5f76a6a281b85732798364ef  db-4.8.30.NC.tar.gz' | sha256sum -c
 # -> db-4.8.30.NC.tar.gz: OK
 tar -xzvf db-4.8.30.NC.tar.gz
-	cd build_unix/
-	../dist/configure --enable-cxx
-	make
-	sudo make install
 
+# Build the library and install to our prefix
+cd db-4.8.30.NC/build_unix/
+#  Note: Do a static build so that it can be embedded into the exectuable, instead of having to find a .so at runtime
+../dist/configure --enable-cxx --disable-shared --with-pic --prefix=$BDB_PREFIX
+make install
+
+# Configure Bitcoin Core to use our own-built instance of BDB
+cd $BITCOIN_ROOT
+./configure (other args...) LDFLAGS="-L${BDB_PREFIX}/lib/" CPPFLAGS="-I${BDB_PREFIX}/include/"
+```
+
+**Note**: You only need Berkeley DB if the wallet is enabled (see the section *Disable-Wallet mode* below).
 
 Boost
 -----