[contrib] Use BDB_LIBS/CFLAGS in install_db4

Author: fanquake

contrib/install_db4.sh

@@ -83,4 +83,4 @@ echo
 echo 'When compiling bitcoind, run `./configure` in the following way:'
 echo
 echo "  export BDB_PREFIX='${BDB_PREFIX}'"
-echo '  ./configure LDFLAGS="-L${BDB_PREFIX}/lib/" CPPFLAGS="-I${BDB_PREFIX}/include/" ...'
+echo '  ./configure BDB_LIBS="-L${BDB_PREFIX}/lib -ldb_cxx-4.8" BDB_CFLAGS="-I${BDB_PREFIX}/include" ...'