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