title: PATH and exec-path set, but emacs does not find executable
body: <p>My .emacs contains</p>
  
  <pre><code>(setenv "PATH" (concat ".:/usr/texbin:/opt/local/bin" (getenv "PATH")))
  (setq exec-path (append exec-path '(".:/usr/texbin:/opt/local/bin")))
  
  (add-to-list 'load-path "/usr/local/share/emacs/site-lisp")
  (require 'tex-site)
  (load "auctex.el" nil t t)
  (load "preview-latex.el" nil t t)
  </code></pre>
  
  <p>/usr/texbin is where latex/pdflatex/.. are located.
  /opt/local/bin/ is where gs can be found.</p>
  
  <p>And yet when I run preview-at-point, which apparently needs both latex and gs, I get</p>
  
  <pre><code>Preview-DviPS finished at Thu Dec 22 11:25:46
  DviPS sentinel: Searching for program: No such file or directory, gs
  </code></pre>
  
  <p>which means that latex could be found all right, but not gs.</p>
  
  <p>I am not sure whether setting exec-path is necessary, perhaps PATH is enough, but I've set it as a debugging measure.</p>
  
  <p>Why can emacs not find gs even though the directory it's in is in both PATH and exec-path?</p>