unless main thread is already exited. Otherwise main thread could be
wrongly interrupted when it uses rb_thread_call_without_gvl(). Patch
by Takehiro Kubo. [Bug #11237][ruby-dev:49044][GH-898]
* test/-ext-/gvl/test_last_thread.rb: new test for the above fix.
* ext/-test-/gvl/call_without_gvl/call_without_gvl.c: new ext for the
above test.
* ext/-test-/gvl/call_without_gvl/extconf.rb: ditto.
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@50900 b2dd03c8-39d4-4d8f-98ff-823fe69b080e