[CMake] Enable policy CMP0056

[phosek]

Summary

When this policy is enabled, CMake honors link flags in try_compile().
This is useful when cross-compiling since the CMake checks will use
the value of CMAKE_EXE_LINKER_FLAGS when linking the test executables.

[smeenai]

Hmm, I'd done this as part of https://reviews.llvm.org/D39431, but then https://reviews.llvm.org/D50407 erroneously removed it.

[smeenai]

Actually, this might not be needed? https://cmake.org/cmake/help/v3.4/manual/cmake-policies.7.html says

The cmake_minimum_required() command does more than report an error if a too-old version of CMake is used to build a project. It also sets all policies introduced in that CMake version or earlier to NEW behavior.

[steveire]

Indeed, the cmake_minimum_required command sets policies to new.

It is still possible that there is another use of cmake_minimum_required in the buildsystem somewhere (which would be an error if reachable from this one).

Anyway, it seems that this patch should not be applied.

llvm/CMakeLists.txt

 # See docs/CMake.html for instructions about how to build LLVM with CMake.
 
 cmake_minimum_required(VERSION 3.4.3)
 
+if(POLICY CMP0056)
+  cmake_policy(SET CMP0056 NEW)
+endif()
+
 if(POLICY CMP0068)
   cmake_policy(SET CMP0068 NEW)
   set(CMAKE_BUILD_WITH_INSTALL_NAME_DIR ON)
 endif()
 
 if(POLICY CMP0075)
   cmake_policy(SET CMP0075 NEW)
 endif()