Guard all accesses to RubyVM::MJIT with defined?(RubyVM::MJIT) &&

* Otherwise those tests, etc cannot run on alternative Ruby implementations.

Author: eregon

test/net/http/test_httpresponse.rb

@@ -78,7 +78,7 @@ def test_read_body_block
 
   def test_read_body_block_mod
     # http://ci.rvm.jp/results/trunk-mjit-wait@silicon-docker/3019353
-    skip 'too unstable with --jit-wait, and extending read_timeout did not help it' if RubyVM::MJIT.enabled?
+    skip 'too unstable with --jit-wait, and extending read_timeout did not help it' if defined?(RubyVM::MJIT) && RubyVM::MJIT.enabled?
     IO.pipe do |r, w|
       buf = 'x' * 1024
       buf.freeze