CMakeLists.txt: Set CMake policy CMP0054 to NEW.

This prevents the following warnings upon a second cmake run:

  CMake Warning (dev) at /usr/share/cmake-3.10/Modules/CMakePushCheckState.cmake:65 (if):
    Policy CMP0054 is not set: Only interpret if() arguments as variables or
    keywords when unquoted.  Run "cmake --help-policy CMP0054" for policy
    details.  Use the cmake_policy command to set the policy and suppress this
    warning.
    Quoted variables like "HAVE_CXX_ATOMICS_WITHOUT_LIB" will no longer be
    dereferenced when the policy is set to NEW.  Since the policy is not set
    the OLD behavior will be used.
  Call Stack (most recent call first):
    CMakeLists.txt:123 (cmake_push_check_state)
    CMakeLists.txt:139 (check_working_cxx_atomics)
  This warning is for project developers.  Use -Wno-dev to suppress it.