python: fixed cross-compiling by setting ac_cv_file__dev_ptmx=yes

Note

We can safely assume that the runtime system will have /dev/ptmx!
Strangely enough on a native build I have /dev/ptmx didn't got detected,
which is most probably an issue with our sandbox and needs some more
observation!

python/python/python.conf

@@ -55,4 +55,6 @@ fix_python_for_64bit() {
 # cross-compiling tweaks
 if atstage cross; then
 	var_append configcache ' ' "ac_cv_buggy_getaddrinfo=no"
+	# on a running system we have /dev/ptmx
+	var_append configcache ' ' "ac_cv_file__dev_ptmx=yes"
 fi