stackbuilder 2.0/2.1 option for setting new PostGIS GDAL Raster variables

[robe]

This is more of a note -- in 2.0.6 and 2.1.3 I added extra dialogs to windows installer to set the environment variables:

It's not that elegant - If you say yes to the two new prompts it sets the system variables:

POSTGIS_GDAL_ENABLED_DRIVERS=GTiff PNG JPEG GIF XYZ DTED USGSDEM AAIGrid

and POSTGIS_ENABLE_OUTDB_RASTERS=1

If you say no it doesn nothing.

Ideally I probably should have allowed them to override the drivers but I figure I'm going to take this out when 2.2 comes around anyway and have them use GUCs though I may set the GUCs for sample database in which case I'd only ask for the prompts if they choose that option.