added support for POSTGIS_GDAL_ENABLED_DRIVERS and POSTGIS_ENABLE_OUTDB_RASTERS environment variables to set the boot values of the GUCs postgis.gdal_enabled_drivers and postgis.enable_outdb_rasters, respectively

git-svn-id: http://svn.osgeo.org/postgis/trunk@12494 b70326c6-7e19-0410-871a-916f4a2858ee
add text mentioning env variables

git-svn-id: http://svn.osgeo.org/postgis/trunk@12495 b70326c6-7e19-0410-871a-916f4a2858ee