Add system gtest

This is the "bad" patch :)

Anyway this needs to be tested, in debian I tweaked the rules file to issue this command
mkdir -p build/gtest && cd build/gtest && cmake /usr/src/gtest && make
this is how to dynamically build gtest on debian, because google (upstream) oppose the build of a shared system library.

What do you think? I'm testing it right now, the build process is somewhat sloooooow