title: I cannot open gdb using M-x gdb on my mac
body: <p>My OS is OSX 10.9.4, and I'm using gdb 7.7.1.</p>
  
  <p>I can open gdb in shell mode in emacs.
  but cannot open gdb using <code>M-x gdb</code>. Emacs just says "No such file or directory, gdb". Why does this happen? Do I need to configure the M-x command?</p>