test/mkmf: $extmk in parallel test

* test/mkmf/base.rb, test/mkmf/test_config.rb: needs to set $extmk
  since worker processes in parallel test mode.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@41771 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

test/mkmf/base.rb

@@ -1,3 +1,5 @@
+$extmk = true
+
 require 'test/unit'
 require 'mkmf'
 require 'tmpdir'

test/mkmf/test_config.rb

@@ -1,3 +1,5 @@
+$extmk = true
+
 require 'test/unit'
 require 'mkmf'
 require_relative '../ruby/envutil'