CMake: look for system gtest

[nim65s]

Hi,

This add a simple test checking if google test is already available on the system.

Because when this is the case, there is no point fetching it from github, especially since this require a working network access and might fail.

On my current use case, the Nix package manager build softwares in a sandbox without network access to ensure their reproducibility, so this it is required to have all dependencies available before the configuration of one software.