FindPostgreSQL: automatically find installations from yum.postgresql.org

In commit v3.3.0-rc1~4^2 (FindPostgreSQL: Search some more common
packaging locations) the PostgreSQL_ADDITIONAL_SEARCH_PATHS variable was
removed. This was used e.g. by osm2pgsql to be able to build on CentOS
with recent PostgreSQL versions. At least add those locations from
yum.postgresql.org, which is a more or less official location.
Status Name Job ID Coverage
  External
passed kwrobot-branch-check