Fix compatibility with older versions of CMake

The "SONAME_FLAG" was introduced in CMake 2.8.9, while the current CMake scripts require 2.8.4 (first line of CMakeLists.txt). Increasing the required CMake version will block building in less recent OSes (e.g., for Ubuntu 12.04 LTS, the latest version from the official repositories is currently 2.8.7). The more compatible solution will be to use the old variable name, i.e. the one which was used before CMake was patched.

NB: this issue is triggered upon specifying -DBUILD_SHARED_LIBS=ON.

Related bug report: http://public.kitware.com/Bug/view.php?id=13884 
Commit which implemented this new variable: http://cmake.org/gitweb?p=cmake.git;a=commitdiff;h=e1409ac5