add more instruction for windows, ubuntu, debian users on setting environment variables.

git-svn-id: http://svn.osgeo.org/postgis/branches/2.1@12508 b70326c6-7e19-0410-871a-916f4a2858ee
note about restart

git-svn-id: http://svn.osgeo.org/postgis/branches/2.1@12509 b70326c6-7e19-0410-871a-916f4a2858ee