[ng] requirements: require GNU make >= 3.81

[ng] requirements: require GNU make >= 3.81

Our implementation of make variables memoization doesn't work GNU make
3.80.  Since GNU make 3.81 has been released by more than six years now,
we require it as the minimal supported version, to avoid wasting efforts
on older versions used only by a vanishingly small percentage of the user
base.

See: <http://lists.gnu.org/archive/html/automake-ng/2012-05/msg00102.html>

* NG-NEWS: Update.
* lib/am/header-vars.am: Error out if the .FEATURES special variable
(introduced only in GNU make 3.81) is not defined.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>