* CMake: simplify FindGMP.cmake
Remove printing of all the different variables, and let FPHSA output the library
name. Add an imported target, which bundles the library and the include
directories for easier usage.
* fix build: vector::c_ptr() now is vector::data()
* CMake: use Threads::Threads imported module
Otherwise the setting of THREADS_PREFER_PTHREAD_FLAG has no effect.
* CMake: remove needless policy setting
The minimum required version is CMake 3.4, where these policies are already set
to new because they were introduced earlier.
* CMake: remove needless variable expansion