The vars.hpp file is included in the conf module and the Vars class is
wrapped in the conf module. VarsWeakPtr was packaged in the base module.
Which will bring problems in the following changes.

So the VarsWeakPtr wrapper has been moved to the conf module.

The original `Vars::detect_release` returns `std::unique_ptr<std::string>`
This makes it complicated for wrapping into other languages.

The modified version returns `char *` - a standard C string.

Tests in Python, Ruby, Perl.

It only tests the situation where a release cannot be detected.
To detect an installed release, a system database with the corresponding
installed packages is required.