Silence a warning about VERSION for CMake >= 3

When using CMake >=3 the following warning pop up

  CMake Warning (dev) at CMakeLists.txt:11 (project):
     Policy CMP0048 is not set: project() command manages VERSION variables.
     Run "cmake --help-policy CMP0048" for policy details.  Use the cmake_policy
     command to set the policy and suppress this warning.

     The following variable(s) would be set to empty:

       PCL_VERSION
   This warning is for project developers.  Use -Wno-dev to suppress it.

This is due to the fact that CMake version 3.0 introduced the VERSION
option to the project() command. In order to *not* use this feature and
not get annoying warnings, the policy CMP0048 has to be explicitly set
to "OLD".
Merge pull request #750 from taketwo/fix-cmake3-warning

Silence a warning about VERSION for CMake >= 3