CMake: use new policy for CMP0051

[t.p.northover]

Summary

I've recently started being annoyed by deprecation warnings for this CMake policy when configuring (apparently CMake 3.11.4 onwards does this) so I decided to try using the new behaviour to see what happens.

Near as I can tell nothing actually relies on the old policy. The original review (https://reviews.llvm.org/D7870) mentioned lldb, but I've built that (and everything else I can get my hands on) with both Linux & macOS and there were no extra warnings or other problems.

So, good to commit? Or does anyone know of something I should actually fix?

[labath]

I am not sure what kinds of problems this can cause, but I can volunteer to help with the lldb side of things in case of any fallout.

[MatzeB]

Looking through llvms macros I see some get_property(... SOURCES) in AddLLVM.cmake. Basically looking for entries in the list that end in a .c or .cpp suffix, which at a first glance looks like it should still work even with some generator expressions in between.

[smeenai]

@beanz did this in https://reviews.llvm.org/rL340436

[beanz]

Heh... yea, didn't notice this review. I removed the policy setting (which will default to NEW). @chapuni fixed the underlying issue that required us to have this enabled *years* ago.

llvm/CMakeLists.txt

 # See docs/CMake.html for instructions about how to build LLVM with CMake.
 
 cmake_minimum_required(VERSION 3.4.3)
 
 cmake_policy(SET CMP0022 NEW)
 
 cmake_policy(SET CMP0048 NEW)
 
-# CMake 3.1 and higher include generator expressions of the form
-# $<TARGETLIB:obj> in the SOURCES property.  These need to be
-# stripped everywhere that access the SOURCES property, so we just
-# defer to the OLD behavior of not including generator expressions
-# in the output for now.
-cmake_policy(SET CMP0051 OLD)
+cmake_policy(SET CMP0051 NEW)
 
 cmake_policy(SET CMP0056 NEW)
 
 cmake_policy(SET CMP0057 NEW)
 
 if(POLICY CMP0068)
   cmake_policy(SET CMP0068 NEW)
   set(CMAKE_BUILD_WITH_INSTALL_NAME_DIR ON)