Provide some instructions to guide the user on how to get a more up-to-date CMake in CMake error message

[mehdi_amini]

Summary

This will display:

CMake Error at CMakeLists.txt:18 (message):
   Your CMake version is 3.17.0. The minimum version of CMake  required to build LLVM is now 3.13.4.

   You can download CMake from: https://github.com/Kitware/CMake/releases
   or can locally get cmake in the current build directory:
   $ wget https://github.com/Kitware/CMake/releases/download/v3.18.0/cmake-3.18.0-Linux-x86_64.tar.gz
   $ echo "4d9a9d3351161073a67e49366d701b6fa4b0343781982dc5eef08a02a750d403  cmake-3.18.0-Linux-x86_64.tar.gz" | sha256sum -c
   $ tar -xf cmake-3.18.0-Linux-x86_64.tar.gz && (cd cmake-3.18.0-Linux-x86_64 && ./bootstrap && make)
   # Done, cmake is usable, nothing is installed on the system, everything is self-contained *inside* the build directory itself.
   $ ./cmake-3.18.0-Linux-x86_64/bin/cmake ../llvm/ -D.... # build LLVM as usual.

[mehdi_amini]

(remove duplicated newline)

[ctetreau]

I think a change such as this would be an improvement. However, I think the linux special case is encouraging bad security habits "wget this random binary and run it!", and IMO should be removed.

I would also provide the download link from cmake.org rather than github. This is more robust in the face of kitware changing their repo hosting site (the github repo that you linked is not even the real repo, it's a mirror)

[mehdi_amini]

In D83995#2158804, @ctetreau wrote:

I think a change such as this would be an improvement. However, I think the linux special case is encouraging bad security habits "wget this random binary and run it!", and IMO should be removed.

Do you mean the non-linux special case? The linux special case does not download a binary but the sources, and also explicitly check the checksum.

I would also provide the download link from cmake.org rather than github. This is more robust in the face of kitware changing their repo hosting site (the github repo that you linked is not even the real repo, it's a mirror)

Look good, I'll update to https://cmake.org/download/

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(FATAL_ERROR
-    "Your CMake version is ${CMAKE_VERSION}. The minimum version of CMake "
-    "required to build LLVM is now 3.13.4.")
+  set(ERROR_MSG " Your CMake version is ${CMAKE_VERSION}. The minimum version of CMake "
+    " required to build LLVM is now 3.13.4.\n\n"
+    " You can download CMake from: https://github.com/Kitware/CMake/releases")
+  if(${CMAKE_HOST_SYSTEM_NAME} STREQUAL "Linux")
+     string(APPEND ERROR_MSG "\n"
+      " or can locally get cmake in the current build directory:\n"
+      " $ wget https://github.com/Kitware/CMake/releases/download/v3.18.0/cmake-3.18.0-Linux-x86_64.tar.gz\n"
+      " $ echo \"4d9a9d3351161073a67e49366d701b6fa4b0343781982dc5eef08a02a750d403  cmake-3.18.0-Linux-x86_64.tar.gz\" | sha256sum -c\n"
+      " $ tar -xf cmake-3.18.0-Linux-x86_64.tar.gz && (cd cmake-3.18.0-Linux-x86_64 && ./bootstrap && make)\n"
+      " # Done, cmake is usable, nothing is installed on the system, everything is self-contained *inside* the build directory itself.\n"
+      " $ ./cmake-3.18.0-Linux-x86_64/bin/cmake ../llvm/ -D.... # build LLVM as usual.")
+  endif()
+  message(FATAL_ERROR ${ERROR_MSG})
 endif()
 
 if(POLICY CMP0068)
   cmake_policy(SET CMP0068 NEW)
   set(CMAKE_BUILD_WITH_INSTALL_NAME_DIR ON)
 endif()
 
 if(POLICY CMP0075)