[cmake] Set CMP0075 to NEW

[smeenai]

Summary

Make the check_include_file* macros honor CMAKE_REQUIRED_LIBRARIES. This
shouldn't cause any of the configuration checks to give different
results (and I did clean configures before and after this change and
confirmed that the resulting CMake caches were identical, though of
course that's just one machine). This suppresses a warning when building
with CMake 3.12 or later.

Unfortunately, this doesn't suppress the warning in clang, because clang
does its own cmake_minimum_required call even when being built in-tree,
and that resets all policy settings.

[smeenai]

Somewhat orthogonal to this diff, but should we change clang so that it only does its own cmake_minimum_required call when it's being built standalone? There's potential for inconsistent policies between building clang in-tree and standalone if we do that, but it also seems strange to just wholesale ignore LLVM's policy settings.

[beanz]

LGTM.

My biggest argument against changing how clang does things is that we want to get the policy failures there too so that we know to fix them. We do support building clang out of tree, and hiding the warnings for in-tree builds will just make it harder to find and fix them.

[smeenai]

That's fair. I can just change the policy for clang explicitly in a separate diff then.

llvm/trunk/CMakeLists.txt

 # See docs/CMake.html for instructions about how to build LLVM with CMake.
 
 cmake_minimum_required(VERSION 3.4.3)
 
 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()
+
 if(NOT DEFINED LLVM_VERSION_MAJOR)
   set(LLVM_VERSION_MAJOR 8)
 endif()
 if(NOT DEFINED LLVM_VERSION_MINOR)
   set(LLVM_VERSION_MINOR 0)
 endif()
 if(NOT DEFINED LLVM_VERSION_PATCH)
   set(LLVM_VERSION_PATCH 0)