Fix issues with GSETTINGS_CHECK_RULE

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

gio/tests/gschema-compile.c

@@ -23,7 +23,7 @@ test_schema (gpointer data)
       gchar *argv[] = {
         "../glib-compile-schemas",
         "--dry-run",
-        "--schema-files", path,
+        "--schema-file", path,
         (gchar *)test->opt,
         NULL
       };