title: When a gem fails, where do I find the mkmf.log file?
body: <blockquote>
  <p>Can't find the 'libpq-fe.h header</p>
  <p>** extconf.rb failed **</p>
  <p>Could not create Makefile due to some reason, probably lack of necessary libraries and/or headers.  Check the
  mkmf.log file for more details.</p>
  </blockquote>
  <p>Where do I find the mkmf.log file?</p>