Add GNU Make 4.0+ version check to Makefile

- Require GNU Make 4.0 or later for $(file ...) function support
- Display helpful error message on macOS directing users to use 'gmake'
- Fixes issue where VERSION_STRING was empty with old make 3.81
- Ensures version numbers appear in dist filenames (e.g., fujinet-lib-apple2-4.8.0.zip)

Users on macOS should use 'gmake' instead of 'make' for proper version handling.

Author: dillera

Makefile

@@ -3,6 +3,11 @@
 # Set the TARGETS and PROGRAM values as required.
 # See makefiles/build.mk for details on directory structure for src files and how to add custom extensions to the build.
 
+# Require GNU Make 4.0 or later for $(file ...) function
+ifeq ($(filter 4.%,$(MAKE_VERSION)),)
+    $(error This Makefile requires GNU Make 4.0 or later. You are using $(MAKE_VERSION). Please use 'gmake' instead of 'make' on macOS.)
+endif
+
 TARGETS = apple2 apple2enh atari c64 coco
 PROGRAM := fujinet.lib