Rename gschema-compile to glib-compile-schemas

And clean up the autofoo a bit: use an uppercase variable,
check for pkg-config, check for presence of glib-compile-schemas.

gio/tests/gschema-compile.c

@@ -21,7 +21,7 @@ test_schema (gpointer data)
       gchar *filename = g_strconcat (test->name, ".gschema.xml", NULL);
       gchar *path = g_build_filename (SRCDIR, "schema-tests", filename, NULL);
       gchar *argv[] = {
-        "../gschema-compile",
+        "../glib-compile-schemas",
         "--dry-run",
         "--schema-files", path,
         (gchar *)test->opt,