[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>

diff --git a/lib/am/header-vars.am b/lib/am/header-vars.am
index 10ff773ab983044707828c9a6c5cf83ed046f5cc..5cd86f6ba029a8edb1d579c0da8b91f27647bc1f 100644 (file)
--- a/lib/am/header-vars.am
+++ b/lib/am/header-vars.am
@@ -16,6 +16,13 @@
 
 VPATH = @srcdir@
 
+## Makefiles generated by Automake-NG require GNU make >= 3.81.
+## The .FEATURES special variable has been introduced in that make
+## version, so use it as a witness to determine whether the current
+## make is good enough.
+$(if $(.FEATURES),, \
+  $(error Automake-NG based builds require GNU make 3.81 or later))
+
 ## The 'all' target must be the default one, independently from the
 ## position it is declared in the output Makefile.
 .DEFAULT_GOAL := all