Use postgis_scripts_installed to fetch installed version if lib_version fails

Soft-uprading from 1.0 to any other 1.x would still fail unless the
old 1.0 library is around. See #2510

git-svn-id: http://svn.osgeo.org/postgis/trunk@12053 b70326c6-7e19-0410-871a-916f4a2858ee