tcltklib.c: rb_thread_alive_p macro

* ext/tk/tcltklib.c (rb_thread_alive_p): turn into a macro, so that
  same code is generated.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39986 b2dd03c8-39d4-4d8f-98ff-823fe69b080e