CMake: add a default root file

Add a new CMakeFiles.txt at top-level to ease build
CMake: allow to build with a provided version string file

Previously, CMake relies on git or a git-tag.txt file to find version info. This patch allow to use a provided src/version_string.hpp header and extract version from this file.
This is useful to build OpenCAMLib from a tarball or a build system like PPA (ie. without fetching OpenCAMLib from GitHub)