[6.0] 0e3471116 Python 3 takes priority over python 2

[Dridi Boukelmoune]

commit 0e3471116ede70e20d01d0ca3694e1990dfe61cb
Author: Federico G. Schwindt <fgsch at lodoss.net>
Date:   Wed Sep 26 07:03:28 2018 -0600

    Python 3 takes priority over python 2
    
    Take 2. Let's see if this time sticks.

diff --git a/varnish.m4 b/varnish.m4
index 8a92d5f10..71df96d9c 100644
--- a/varnish.m4
+++ b/varnish.m4
@@ -129,7 +129,9 @@ AC_DEFUN([_VARNISH_CHECK_DEVEL], [
 # _VARNISH_CHECK_PYTHON
 # ---------------------
 AC_DEFUN([_VARNISH_CHECK_PYTHON], [
-
+	m4_define_default([_AM_PYTHON_INTERPRETER_LIST],
+[python3.9 python3.8 python3.7 python3.6 python3.5 python3.4 python2.7 dnl
+python python2 python3])
 	AM_PATH_PYTHON([2.7], [], [
 		AC_MSG_ERROR([Python >= 2.7 is required.])
 	])