fixing FindGlew on mavericks

the pull-req https://github.com/PointCloudLibrary/pcl/pull/529 requires following fix:
1. `CMAKE_SYSTEM_VERSION` instead of `CMAKE_VERSION_SYSTEM` [cmake doc](http://www.cmake.org/cmake/help/v2.8.12/cmake.html#variable:CMAKE_SYSTEM_VERSION)
2. `CMAKE_SYSTEM_VERSION` returns `13.0.0` in mavericks
3. use pkg-config to find Glew on mavericks. calling `find_package(Glew)` would be an infinite recursive call.

```sh
$ uname -a
Darwin dhcp204.jsk.t.u-tokyo.ac.jp 13.0.0 Darwin Kernel Version 13.0.0: Thu Sep 19 22:22:27 PDT 2013; root:xnu-2422.1.72~6/RELEASE_X86_64 x86_64
```