fix mac path variable problem

Fix the mac path variable problem. If you don't do this, you $PATH in emacs just contains $HOME(no /usr/bin, /usr/local/bin etc.). You can't use 'make, autoconf, pep8, and any commands that related to the $PATH variable in emacs. You can't even install oh-my-emacs on a mac. You should change the codes' position to suit your style.