[CMake] Enforce the minimum CMake version to be at least 3.13.4

[ldionne]

Summary

This commit changes the warning for CMake < 3.13.4 into a fatal error.
The intent is to revert and re-apply this simple commit until all build
bots are migrated to CMake >= 3.13.4.

This is part of the effort discussed on llvm-dev here:

http://lists.llvm.org/pipermail/llvm-dev/2020-April/140578.html

[ldionne]

This review is for the upcoming CMake upgrade once we branch off the LLVM 11 release branch.

I won't apply it until after the branch has been created, which is around July.

[smeenai]

Hmm, won't you be doing the upgrade by changing the cmake_minimum_required to 3.13.4? At that point, the fatal error would never be triggered, right?

[ldionne]

In D78646#1997438, @smeenai wrote:

Hmm, won't you be doing the upgrade by changing the cmake_minimum_required to 3.13.4? At that point, the fatal error would never be triggered, right?

This patch is meant to be as tiny as possible, because I expect to apply/revert it perhaps several times. The child patch https://reviews.llvm.org/D78648 is the one that will finish the job once this patch finally triggers no more errors.

[smeenai]

Ah, makes sense. LGTM to ship after the branch cut!

[ldionne]

I'm pushing this now folks! Let's see what burns.

llvm/CMakeLists.txt

 # See docs/CMake.html for instructions about how to build LLVM with CMake.
 
 cmake_minimum_required(VERSION 3.4.3)
 
 if ("${CMAKE_VERSION}" VERSION_LESS "3.13.4")
-  message(WARNING
-    "Your CMake version is ${CMAKE_VERSION}. Starting with LLVM 12.0.0, the "
-    "minimum version of CMake required to build LLVM will become 3.13.4, and "
-    "using an older CMake will become an error. Please upgrade your CMake to "
-    "at least 3.13.4 now to avoid issues in the future!")
+  message(FATAL_ERROR
+    "Your CMake version is ${CMAKE_VERSION}. The minimum version of CMake "
+    "required to build LLVM is now 3.13.4.")
 endif()
 
 if(POLICY CMP0068)
   cmake_policy(SET CMP0068 NEW)
   set(CMAKE_BUILD_WITH_INSTALL_NAME_DIR ON)
 endif()
 
 if(POLICY CMP0075)