GuixSD distro detection during configuration

[phodina]

Hi,
what is the best way to detect the GuixSD OS during the build?

I encountered the problem during build of realmd in the configure.ac [1].

Some distros have special file in /etc. So my first idea was to use 
/etc/config.scm. However, this is not visible during the package build.

Therefore I went with the /gnu/store. This works but I'm interested if there 
isn't even a better option.
[1] https://github.com/freedesktop/realmd/blob/master/configure.ac