CMake 3.11 emits "Deprecation Warning at CMakeLists.txt:20" when configuring the project

[s3rvac]

When using CMake 3.11, the cmake command emits a deprecation warning.

Steps to reproduce

• Ensure that you have CMake 3.11 installed in your system.
• Run the following commands:

git clone https://github.com/avast-tl/llvm.git
mkdir -p llvm/build && cd llvm/build
cmake ..

Output

-- No build type selected, default to Debug
CMake Deprecation Warning at CMakeLists.txt:20 (cmake_policy):
  The OLD behavior for policy CMP0051 will be removed from a future version
  of CMake.

  The cmake-policies(7) manual explains that the OLD behaviors of all
  policies are deprecated and that a policy should be set to OLD only under
  specific short-term circumstances.  Projects should be ported to the NEW
  behavior and not rely on setting a policy to OLD.

-- The C compiler identification is GNU 8.1.0
-- The CXX compiler identification is GNU 8.1.0
...

Expected output

The same as when using CMake 3.10:

-- No build type selected, default to Debug
-- The C compiler identification is GNU 8.1.0
-- The CXX compiler identification is GNU 8.1.0
...

We should fix the build system so no warning is emitted as it only clutters the output.