[libc] Guard cmake policy to avoid errors on older versions of cmake.

[PaulkaToast]

Summary

Building with older versions of cmake result in this error.
Error Message:

CMake Error at /src/llvm-project/libc/CMakeLists.txt:5 (cmake_policy):
Policy "CMP0076" is not known to this version of CMake.

The set policy statement must be guarded as described here.
Example of existing cmake policy guarding in llvm.

The policy used here was introduced in version 3.13 after the minimum version supported by llvm which is 3.4.3.

[abrachet]

Do those older versions still build fine without this policy being set?

[PaulkaToast]

In D75027#1888561, @abrachet wrote:

Do those older versions still build fine without this policy being set?

Yes they do because the old behavior is default before the policy existed.

[MaskRay]

A few months ago @beanz asked whether we can upgrade CMake to 3.15 https://lists.llvm.org/pipermail/llvm-dev/2019-October/136295.html It ended up with nothing definite.

Maybe we could make a smaller step, say CMake 3.12.

[sivachandra]

Like @MaskRay suggests, would bumping LLVM-libc's minimum required version instead of this change help/unblock you?

[PaulkaToast]

In D75027#1888604, @sivachandra wrote:

Like @MaskRay suggests, would bumping LLVM-libc's minimum required version instead of this change help/unblock you?

This issue doesn't block me, I just noticed this when building on ubuntu-xenial 16.04 which is from 2016. I just figured that we wanted to remain consistent in what is supported by llvm. https://llvm.org/docs/CMake.html

Version 3.4.3 is the minimum required.

[PaulkaToast]

Abandoned due to recent cmake upgrade: D77781.

libc/CMakeLists.txt

 cmake_minimum_required(VERSION 3.4.3)
 
 # Use old version of target_sources command which converts the source
 # file paths to full paths.
+if(POLICY CMP0076)
   cmake_policy(SET CMP0076 OLD)
+endif()
 list(APPEND CMAKE_MODULE_PATH  "${CMAKE_CURRENT_SOURCE_DIR}/cmake/modules")
 
 # The top-level source directory of libc.
 set(LIBC_SOURCE_DIR ${CMAKE_CURRENT_SOURCE_DIR})
 # The top-level directory in which libc is being built.
 set(LIBC_BUILD_DIR ${CMAKE_CURRENT_BINARY_DIR})
 
 # Path libc/scripts directory.