m4: Try to use "python" as Python2 interpreter only as last resort

This patch tries to find Python 2 as "python2", then "python2.7" and finally
"python".

This is needed since "/usr/bin/python" is used as Python 3 on some Linux
distributions (for example on Arch Linux) and on Fedora 28
"/usr/bin/python" will be deprecated [1]:
    "All scripts shall explicitly use /usr/bin/python2."

[1] https://fedoraproject.org/wiki/FinalizingFedoraSwitchtoPython3

Signed-off-by: Timothy Redaelli <[email protected]>
Signed-off-by: Ben Pfaff <[email protected]>

m4/openvswitch.m4

@@ -356,7 +356,7 @@ AC_DEFUN([OVS_CHECK_PYTHON],
         ovs_cv_python=$PYTHON
       else
         ovs_cv_python=no
-        for binary in python python2.7; do
+        for binary in python2 python2.7 python; do
           ovs_save_IFS=$IFS; IFS=$PATH_SEPARATOR
           for dir in $PATH; do
             IFS=$ovs_save_IFS