Enforce GNU make 4+ at Makefile parse time

[Copilot]

The root Makefile previously only guarded for GNU make presence. This update tightens the preflight check to require GNU make major version 4 or newer and fail before target evaluation.

• Preflight guard hardening

  • Kept the GNU-make detection via MAKE_VERSION origin.
  • Updated the missing-GNU-make error to explicitly call out undefined MAKE_VERSION.

• Minimum version enforcement

  • Added parse-time extraction of MAKE_MAJOR from MAKE_VERSION.
  • Added an early failure path for major versions 0..3 using GNU make-native functions (no shell arithmetic dependency).

• Failure messaging

  • Error messages now distinguish:
    • GNU make not detected (MAKE_VERSION undefined)
    • GNU make detected but version too old (shows detected version)

ifeq ($(origin MAKE_VERSION),undefined)
  $(error GNU make >= 4 is required. MAKE_VERSION is undefined, so GNU make may not be installed or in use. Run 'make --version' to verify.)
endif

MAKE_MAJOR := $(firstword $(subst ., ,$(MAKE_VERSION)))
ifneq ($(filter 0 1 2 3,$(MAKE_MAJOR)),)
  $(error GNU make >= 4 is required. Found version $(MAKE_VERSION). Run 'make --version' to check your current version.)
endif

[Copilot]

Pull request overview

This PR strengthens the repository’s root Makefile preflight checks so that GNU make is not only detected, but also required to be major version 4+, and that failure occurs during Makefile parsing (before any targets are evaluated).

Changes:

• Adds an early parse-time error when MAKE_VERSION is not defined (intended to indicate GNU make is not in use).
• Extracts MAKE_MAJOR from MAKE_VERSION at parse time and errors out for major versions 0..3.
• Updates error messaging to distinguish “GNU make not detected” vs “GNU make too old”.

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