acinclude: Enable building for Linux kernel 4.15

Signed-off-by: Greg Rose <gvrose8192@gmail.com>
Acked-by: Pravin B Shelar <pshelar@ovn.org>

acinclude.m4

@@ -151,10 +151,10 @@ AC_DEFUN([OVS_CHECK_LINUX], [
     AC_MSG_RESULT([$kversion])
 
     if test "$version" -ge 4; then
-       if test "$version" = 4 && test "$patchlevel" -le 13; then
+       if test "$version" = 4 && test "$patchlevel" -le 15; then
           : # Linux 4.x
        else
-          AC_ERROR([Linux kernel in $KBUILD is version $kversion, but version newer than 4.13.x is not supported (please refer to the FAQ for advice)])
+          AC_ERROR([Linux kernel in $KBUILD is version $kversion, but version newer than 4.15.x is not supported (please refer to the FAQ for advice)])
        fi
     elif test "$version" = 3 && test "$patchlevel" -ge 10; then
        : # Linux 3.x