For policy-specific tests, use the version before the policy was
introduced. Otherwise, use 3.5 where possible.
Also, remove `cmake_minimum_required()` and `project()` calls from
individual cases where they are handled by `CMakeLists.txt`.
Starting with Xcode 7 the OSX and iOS SDKs contain only stub
files for dynamic system libraries. These stub files contain
some meta data and a list of exported sysbols in plain text.
They are handled by the toolchain like regular dylibs.