FindPostgreSQL: automatically find installations from yum.postgresql.org

In commit d4fd30d8 (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.

The variable can be replaced downstream with CMAKE_PREFIX_PATH, which will work both with newer and older versions of CMake, so I do not see a need to reintroduce them.