Fix logic to detect vs120
MJIT_CC seems not defined https://ci.appveyor.com/project/ruby/ruby/builds/32161572/job/u5sw8yn4in87heki
Fix logic to detect vs120
MJIT_CC seems not defined
https://ci.appveyor.com/project/ruby/ruby/builds/32161572/job/u5sw8yn4in87heki