* 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
`contrib/cmake/bootstrap.py` script no longer needs to be executed.
The previous location of the CMake files was a compromise proposed
by @agurfinkel in #461. While this has served us well (allowing progress
to be made) over time limitations of this approach have appeared.
The main problem is that doing many git operations (e.g. pull, rebase)
means the CMake files don't get updated unless the user remembers to
run the script. This can lead to broken and confusing build system
behaviour.
This commit only does the file moving and necessary changes to
`.gitignore`. Other changes will be done in subsequent commits.
