Fix empty Cache HELPSTRING parsing error on OS X 10.10 Yosemite.

- Without quotations, on Yosemite, CMake will fail on first configure,
  but will then work fine subsequently.  This is because if the
  HELPSTRING property is empty, without "" quotes, it is removed from
  the arguments list for set().

Change-Id: Ia60444695e9770b947f145a54e949c9eeae67fbb
1 file changed