cmake: set CMP0054 to NEW to avoid variables over expansion (since cmake 3.8)

cmake: set CMP0054 to NEW to avoid variables over expansion (since cmake 3.8)

==> win: CMake Warning (dev) at CMakeLists.txt:782 (elseif):
==> win:   Policy CMP0054 is not set: Only interpret if() arguments as variables or
==> win:   keywords when unquoted.  Run "cmake --help-policy CMP0054" for policy
==> win:   details.  Use the cmake_policy command to set the policy and suppress this
==> win:   warning.
==> win:
==> win:   Quoted variables like "MSVC" will no longer be dereferenced when the policy
==> win:   is set to NEW.  Since the policy is not set the OLD behavior will be used.
==> win: This warning is for project developers.  Use -Wno-dev to suppress it.