Ignore warnings in external modules when using CMa

问题:

I am using CMake GUI (no version) with CMake 3.6.1. I am using an external module with add_subdirectory that shows me some warnings that I do not like (because of the annoying pollution):

CMake Warning (dev) at D:/Sources/.../external/g3log/latest/Build.cmake:11 (IF):
  Policy CMP0054 is not set: Only interpret if() arguments as variables or
  keywords when unquoted.  Run "cmake --help-policy CMP0054" for policy
  details.  Use the cmake_policy command to set the policy and suppress this
  warning.

  Quoted variables like "MSVC" will no longer be dereferenced when the policy
  is set to NEW.  Since the policy is not set the OLD behavior will be used.
Call Stack (most recent call first):
  D:/Sources/.../external/g3log/latest/CMakeLists.txt:72 (INCLUDE)
This warning is for project developers.  Use -Wno-dev to suppress it.

I want to hide these warnings without touching the external files. -Wno-dev would be ok if it will affect only the external module (g3log).

I tried using cmake_policy like following with no effect:

cmake_policy(PUSH)
cmake_policy(SET CMP0054 OLD)
add_subdirectory(${g3log_DIR} ${CMAKE_BINARY_DIR}/../g3log)
cmake_policy(POP)

回答1:

Turning my comments into an answer

That sounds like your external module does have a project() command. This resets the policies for this sub-module and below.

To demonstrate a possible solution, lets say you have a external project as the following:

g3log/CMakeLists.txt

cmake_minimum_required(VERSION 2.8)
project(g3log NONE)

set(VAR1 "Hello World")
set(VAR2 "VAR1")
if ("${VAR2}" STREQUAL "${VAR1}")
    message("CMP0054 old behavior")
endif()

You can now set CMAKE_POLICY_DEFAULT_CMP0054 to OLD (or even better to NEW; nobody really wanted the "OLD" behavior) to get rid of the "Policy CMP0054 is not set" warnings you will get with newer versions of CMake:

CMakeLists.txt

cmake_minimum_required(VERSION 3.1)
project(PolicyOverwrite NONE)

set(CMAKE_POLICY_DEFAULT_CMP0054 NEW)
add_subdirectory(g3log)

Now you have set a default for policy CMP0054 to be used if none is explicitly given in your project or one of the external projects you are using.