[CMake] Patch proposal for Purify under Windows

[Judicaël Bedouet]

Hello,

I made a patch to improve integration between Purify and CTest under Windows.
http://public.kitware.com/Bug/view.php?id=8153

This patch corrects a bug under Windows (CTest was unable to analyse
Purify output) and allows to specify a suppression file (via
/FilterFiles option).

Have a good day,

J. Bedouet