Modify Berkeley DB 4.8 install instructions so that it can be installed in a different directory

Test Plan:
Installed BDB 4.8 by pasting the commands into terminal, then typed "make check"
Behaves as expected

Reviewers: freetrader, deadalnix, #bitcoin_abc

Reviewed By: deadalnix, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D563