Fixed #20269 -- Adapted PostGIS template create script for CentOS/RHEL

Thanks Stephane Benchimol for the report and the initial script and
mfandreas for the patch.

docs/ref/contrib/gis/install/create_template_postgis-1.5.sh

 #!/usr/bin/env bash
-POSTGIS_SQL_PATH=`pg_config --sharedir`/contrib/postgis-1.5
+if [[ `uname -r | grep el6` ]]; then
+  POSTGIS_SQL_PATH=`pg_config --sharedir`/contrib/postgis
+  POSTGIS_SQL_FILE=$POSTGIS_SQL_PATH/postgis-64.sql
+else
+  POSTGIS_SQL_PATH=`pg_config --sharedir`/contrib/postgis-1.5
+  POSTGIS_SQL_FILE=$POSTGIS_SQL_PATH/postgis.sql
+fi
 createdb -E UTF8 template_postgis # Create the template spatial database.
 createlang -d template_postgis plpgsql # Adding PLPGSQL language support.
 psql -d postgres -c "UPDATE pg_database SET datistemplate='true' WHERE datname='template_postgis';"
-psql -d template_postgis -f $POSTGIS_SQL_PATH/postgis.sql # Loading the PostGIS SQL routines
+psql -d template_postgis -f $POSTGIS_SQL_FILE # Loading the PostGIS SQL routines
 psql -d template_postgis -f $POSTGIS_SQL_PATH/spatial_ref_sys.sql
 psql -d template_postgis -c "GRANT ALL ON geometry_columns TO PUBLIC;" # Enabling users to alter spatial tables.
 psql -d template_postgis -c "GRANT ALL ON geography_columns TO PUBLIC;"