flex: fix build with autoconf 2.70

autoconf 2.70 is coming soon which has some small behavioural fixes, so
backport a patch from upstream to fix the build with that release.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 946610f2)
Signed-off-by: Steve Sakoman <steve@sakoman.com>