[PATCH v4] kbuild: ensure Make >= 3.82 is used

[Masahiro Yamada]

Documentation/process/changes.rst notes the minimal GNU Make version,
but it is not checked anywhere.

We could check $(MAKE_VERSION), but another simple way is to check
$(.FEATURES) since the feature list always grows.

GNU Make 3.81 expands $(.FEATURES) to:
  target-specific order-only second-expansion else-if archives jobserver check-symlink

GNU Make 3.82 expands $(.FEATURES) to:
  target-specific order-only second-expansion else-if shortest-stem undefine archives jobserver check-symlink

To ensure Make >= 3.82, you can check either 'shortest-stem' or
'undefine'.

This way is not always possible. For example, Make 4.0 through 4.2 have
the same set of $(.FEATURES). At that point, we will need to come up
with a different approach.

Signed-off-by: Masahiro Yamada <masahiroy@xxxxxxxxxx>
Reviewed-by: Nathan Chancellor <nathan@xxxxxxxxxx>
Reviewed-by: Nicolas Schier <nicolas@xxxxxxxxx>
---

Changes in v4:
  - Show the minumal required version in the error message
  - More commit description

Changes in v3:
  - Check the version in a different way

 Makefile | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/Makefile b/Makefile
index 591485152a95..7f831e63b39b 100644
--- a/Makefile
+++ b/Makefile
@@ -11,6 +11,10 @@ NAME = Hurr durr I'ma ninja sloth
 # Comments in this file are targeted only to the developer, do not
 # expect to learn how to build the kernel reading this file.
 
+ifeq ($(filter undefine,$(.FEATURES)),)
+$(error GNU Make >= 3.82 is required. Your Make version is $(MAKE_VERSION))
+endif
+
 $(if $(filter __%, $(MAKECMDGOALS)), \
 	$(error targets prefixed with '__' are only for internal use))
 
-- 
2.34.1