build: work around an old version cmake bug.

build: work around an old version cmake bug.

On fedora core 15 with the cmake 2.8.4, Yi experienced a build error.
It turns out that the cmake may handle the file directorys with double
slashs incorrectly when the file is on a target's dependcy list and
be a output file name of a custom command.

This small patch could work around that issue.

Signed-off-by: Zhigang Gong <zhigang.gong@intel.com>
Tested-by: "Sun, Yi" <yi.sun@intel.com>