Disable building examples for imported libraries

Kitty and Mockturtle libraries have options to build examples on by default. Some examples are actually breaking the build under some circumstances (host-compiler pair), bring nothing to the project, and therefore are better to be disabled.

lib/CMakeLists.txt already contains setting do disable Mockturtle examples, but due to how CMake works, it was ignored in the subproject. It is required to explicitely define CMake policy CMP0077 for subprojects with lower cmake_minimum_required to respect the setting.