Handle the fact PostgreSQL_LIBRARIES might be a full path
Handle the fact PostgreSQL_LIBRARIES might be a full path

On debian, with cmake 3.12.1, PostgreSQL_LIBRARIES contains the
full path to libpq.so, resulting in POSTGRESQL_LIB_FILE being set
to lib/usr/.../libpq.so.so.

This patch aims at finding out this situation and using
get_filename_component instead of prepending lib and appending .so
in this situation.

matthijs/sqlpp11-connector-postgresql,matthijs/sqlpp11-connector-postgresql,matthijs/sqlpp11-connector-postgresql