add PGLIBS variable to env files so psql can find needed libraries

[robertdahlem]

General information

Note: This PR depends on #792. The diff includes those changes for now but will clean up once #792 is merged."

It adds an optional PGLIBS variable to the .env file of mk_postgres.py

This is useful when your psql command from PG_BINARY_PATH does not find its libraries like libpq.so.
In this case the command su - $DBUSER -c "$PG_BINARY_PATH ..."
gets expanded to su - $DBUSER -c "LD_LIBRARY_PATH=$PGLIBS $PG_BINARY_PATH ..."

Proposed changes

This patch adds the LD_LIBRARY_PATH=$PGLIBS environment variable to the call of PG_BINARY_PATH when PGLIBS is set.