diff --git a/ext/extmk.rb b/ext/extmk.rb
index 70067d28b8..b0d84ef00f 100755
--- a/ext/extmk.rb
+++ b/ext/extmk.rb
@@ -417,7 +417,6 @@ elsif CONFIG['EXTSTATIC']
 else
   $ruby = '$(topdir)/ruby' + EXEEXT
 end
-$mflags << "BUILTRUBY=#$ruby"
 $ruby = [$ruby]
 $ruby << "-I'$(topdir)'"
 unless CROSS_COMPILING
@@ -428,6 +427,7 @@ end
 topruby = $ruby
 $ruby = topruby.join(' ')
 $mflags << "ruby=#$ruby"
+$builtruby = '$(topdir)/ruby' + EXEEXT # Must be an executable path
 
 MTIMES = [__FILE__, 'rbconfig.rb', srcdir+'/lib/mkmf.rb'].collect {|f| File.mtime(f)}
 
diff --git a/lib/mkmf.rb b/lib/mkmf.rb
index 79bb96ba75..efe3419fd7 100644
--- a/lib/mkmf.rb
+++ b/lib/mkmf.rb
@@ -2076,7 +2076,11 @@ sitearch = #{CONFIG['sitearch']}
 ruby_version = #{RbConfig::CONFIG['ruby_version']}
 ruby = #{$ruby.sub(%r[\A#{Regexp.quote(RbConfig::CONFIG['bindir'])}(?=/|\z)]) {'$(bindir)'}}
 RUBY = $(ruby#{sep})
-BUILTRUBY = $(RUBY)
+BUILTRUBY = #{if defined?($builtruby) && $builtruby
+    $builtruby
+  else
+    File.join('$(bindir)', CONFIG["RUBY_INSTALL_NAME"] + CONFIG['EXEEXT'])
+  end}
 ruby_headers = #{headers.join(' ')}
 
 RM = #{config_string('RM', &possible_command) || '$(RUBY) -run -e rm -- -f'}
diff --git a/tool/fake.rb b/tool/fake.rb
index 88709b2f23..91dfb041c4 100644
--- a/tool/fake.rb
+++ b/tool/fake.rb
@@ -45,6 +45,7 @@ prehook = proc do |extmk|
   $extout_prefix = '$(extout)$(target_prefix)/'
   config = RbConfig::CONFIG
   mkconfig = RbConfig::MAKEFILE_CONFIG
+  $builtruby ||= File.join(builddir, config['RUBY_INSTALL_NAME'] + config['EXEEXT'])
   RbConfig.fire_update!("builddir", builddir)
   RbConfig.fire_update!("buildlibdir", builddir)
   RbConfig.fire_update!("libdir", builddir)