[Misc #22206] Fix snapshot dependency generation

Skip dependency generation when exporting stable branches that
predate the Ruby dependency scanner.  Avoid loading GNU make
definitions into the temporary Makefile so snapshot generation also
works with BSD make.

tool/make-snapshot

@@ -486,7 +486,6 @@ def package(vcs, rev, destdir, tmp = nil)
         end
       end
       mk << commonmk.gsub(/\{\$([^(){}]*)[^{}]*\}/, "").sub(/^revision\.tmp::$/, '\& Makefile')
-      mk << File.read("defs/gmake.mk")
       mk << <<-'APPEND'
 
 update-download:: touch-unicode-files
@@ -503,7 +502,9 @@ touch-unicode-files:
       clean.create(".revision.time")
       ENV["CACHE_SAVE"] = "no"
       make = MAKE.new(args)
-      return unless make.run("fix-depends")
+      if File.file?("tool/mkdepend.rb")
+        return unless make.run("fix-depends")
+      end
       return unless make.run("update-download")
       clean.push("rbconfig.rb", ".rbconfig.time", "enc.mk", "ext/ripper/y.output", ".revision.time")
       Dir.glob("**/*") do |dest|

tool/test/test_mkdepend.rb

@@ -861,7 +861,11 @@ class TestMkdepend < Test::Unit::TestCase
     )
     assert_include(snapshot, 'args["MKDEPEND_FILES"] = "--scope=core"')
     assert_include(snapshot, 'args["MKDEPEND_OPTIONS"] = ""')
-    assert_include(snapshot, 'make.run("fix-depends")')
+    assert_match(
+      %r{if File\.file\?\("tool/mkdepend\.rb"\).*make\.run\("fix-depends"\)}m,
+      snapshot,
+    )
+    assert_not_include(snapshot, 'File.read("defs/gmake.mk")')
   end
 
   def test_file_without_markers_is_not_updated