Fix scons -c and scons --help on Windows.
Fix scons -c and scons --help on Windows.

* SConstruct:
  (opts.AddVariables): Use platform specific default directories.

x-clone/serf,soufianeEL/serf,yesoce/serf,yesoce/serf,soufianeEL/serf,stoneCC/serf,KyleSanderson/serf,zhengcq315/serf,zhengcq315/serf,zhupan/serf,zhengcq315/serf,google-code-export/serf,yesoce/serf,alex1818/serf,pengdu/serf,alex1818/serf,SirJson/serf,soufianeEL/serf,SirJson/serf,lazysoez/serf,pengdu/serf,KyleSanderson/serf,alex1818/serf,zhengcq315/serf,zhupan/serf,KyleSanderson/serf,lazysoez/serf,x-clone/serf,google-code-export/serf,x-clone/serf,SirJson/serf,soufianeEL/serf,stoneCC/serf,google-code-export/serf,lazysoez/serf,SirJson/serf,x-clone/serf,alex1818/serf,stoneCC/serf,stoneCC/serf,zhupan/serf,pengdu/serf,KyleSanderson/serf,pengdu/serf,yesoce/serf,google-code-export/serf,lazysoez/serf,zhupan/serf