build: only update config-extra.h if it changes

This is to avoid updating config-extra.h timestamp very time one touches
Makefile.am, because it has a large dependency chain and makes
debugging of the Makefile inconvenient.
14 jobs for lr/config-extra-h in 50 minutes and 16 seconds (queued for 3 seconds)
latest
Status Job ID Name Coverage
  Test
passed #353855
allowed to fail
checkpatch

00:02:29

manual #353860
allowed to fail manual
t_centos:7.5.1804
passed #353861
t_centos:7.6.1810

00:43:26

passed #353864
t_debian:9

00:34:55

passed #353866
t_debian:sid

00:20:41

manual #353865
allowed to fail manual
t_debian:testing
passed #353856
t_fedora:28

00:47:30

passed #353857
t_fedora:29

00:37:08

passed #353858
t_fedora:30

00:34:51

manual #353859
allowed to fail manual
t_fedora:rawhide
passed #353862
t_ubuntu:16.04

00:21:33

passed #353863
t_ubuntu:18.04

00:21:36

 
  External
failed NetworkManager test verification

failed NetworkManager test verification

05:35:24