Fixes #15236
Two changes were needed:
- pg_config from postgresql package wasn't reporting the correct location for
the pgxs extension system, after the output split
- json_c is now split in dev and out outputs, postgis configure doesn't find the
library location properly
Closes #15470
- support older versions (which are used by other distributions)
- give utilyt scripts nicer names
- pg_db_postgis_enable: support loading comments
svn path=/nixpkgs/trunk/; revision=21992