Added mac platform check before we try to use mac specific config var
Added mac platform check before we try to use mac specific config var

Tyriar/xterm.js,xtermjs/xterm.js,xtermjs/xterm.js,Tyriar/xterm.js,sourcelair/xterm.js,Tyriar/xterm.js,xtermjs/xterm.js,xtermjs/xterm.js,sourcelair/xterm.js,sourcelair/xterm.js,Tyriar/xterm.js,Tyriar/xterm.js,sourcelair/xterm.js,xtermjs/xterm.js