The GSettings schema compiler was accepting any string as a path. It is
probably quite a common mistake to suspect that '/apps/foo' is a valid
path name when this will cause all sorts of trouble later. Check for
this case and report the error.
Rename the --schema-files option to --schema-file, since it only
accepts one file at a time. Change the GSETTINGS_CHECK_RULE to
use it that way, too. And also make it work better with !srcdir
builds.
Bugs #616731 and #616864
