Read GNUmakefile when the build directory has no Makefile

`configure.bat --with-gnu-make` writes only `GNUmakefile`. The tests under `test/mkmf` failed to load with `Errno::ENOENT`, and `spec/default.mspec` lost `srcdir` and ran specs without `tool/runruby.rb`.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

spec/default.mspec

@@ -22,7 +22,8 @@ class MSpecScript
 
   builddir = Dir.pwd
   srcdir = ENV['SRCDIR']
-  srcdir ||= File.read("Makefile", encoding: "US-ASCII")[/^\s*srcdir\s*=\s*(.+)/i, 1] rescue nil
+  makefile = File.exist?("Makefile") ? "Makefile" : "GNUmakefile"
+  srcdir ||= File.read(makefile, encoding: "US-ASCII")[/^\s*srcdir\s*=\s*(.+)/i, 1] rescue nil
   config = RbConfig::CONFIG
 
   # The default implementation to run the specs.

test/mkmf/base.rb

@@ -2,7 +2,9 @@
 $extmk = true
 require 'rbconfig'
 RbConfig.fire_update!("top_srcdir", File.expand_path("../..", __dir__))
-File.foreach(RbConfig::CONFIG["topdir"]+"/Makefile") do |line|
+makefile = RbConfig::CONFIG["topdir"]+"/Makefile"
+makefile = RbConfig::CONFIG["topdir"]+"/GNUmakefile" unless File.exist?(makefile)
+File.foreach(makefile) do |line|
   if /^CC_WRAPPER\s*=\s*/ =~ line
     RbConfig.fire_update!('CC_WRAPPER', $'.strip)
     break