Disable compiler warnings when using FetchContent with CMake [duplicate] - cmake

Question

I have a C/C++ project in which I want to use CppUTest.
So I include the dependency of CppUTest with:
include(FetchContent)
FetchContent_Declare(
CppUTest
GIT_REPOSITORY https://github.com/cpputest/cpputest.git
GIT_TAG latest-passing-build
CONFIGURE_COMMAND ""
BUILD_COMMAND ""
)
set(TESTS OFF CACHE BOOL "Switch off CppUTest Test build")
FetchContent_MakeAvailable(CppUTest)
This works fine.
In my (automotive) project, I use the C++20 standard and I need to enable all/most warnings. But when I do, CppUTest brings me a lot of warnings. So, I would like to compile CppUTest with different compiler options than the project's code.
How can I configure that in CMake?
I googled a lot but didn't find anything that works.
Thanks,
Stefan
PS: My simplified CMake file is:
cmake_minimum_required(VERSION 3.5)
project(projectName LANGUAGES CXX C)
set(CMAKE_C_STANDARD 99)
set(CMAKE_C_STANDARD_REQUIRED ON)
set(CMAKE_CXX_STANDARD 20)
set(CMAKE_CXX_STANDARD_REQUIRED ON)
set(CMAKE_CXX_EXTENSIONS OFF)
if ("${CMAKE_CXX_COMPILER_ID}" MATCHES "Clang")
MESSAGE("Setting Clang flags:")
SET(COMPILE_FLAGS_WARNINGS "-Weverything")
SET(COMPILE_FLAGS_DEBUG "-Og")
SET(COMPILE_FLAGS_RELEASE "-O2")
SET(COMPILE_FLAGS_COVERAGE "--coverage")
elseif ("${CMAKE_CXX_COMPILER_ID}" STREQUAL "GNU")
#more options here
elseif ("${CMAKE_CXX_COMPILER_ID}" STREQUAL "MSVC")
#more options here
endif()
MESSAGE("Debug: ${CMAKE_CXX_FLAGS} ${COMPILE_FLAGS_WARNINGS} ${COMPILE_FLAGS_DEBUG}")
MESSAGE("Release: ${CMAKE_CXX_FLAGS} ${COMPILE_FLAGS_WARNINGS} ${COMPILE_FLAGS_RELEASE}")
MESSAGE("Coverage: ${CMAKE_CXX_FLAGS} ${COMPILE_FLAGS_WARNINGS} ${COMPILE_FLAGS_DEBUG} ${COMPILE_FLAGS_COVERAGE}")
### Dependencies:
include(FetchContent)
FetchContent_Declare(
CppUTest
GIT_REPOSITORY https://github.com/cpputest/cpputest.git
GIT_TAG latest-passing-build
CONFIGURE_COMMAND ""
BUILD_COMMAND ""
)
set(TESTS OFF CACHE BOOL "Switch off CppUTest Test build")
FetchContent_MakeAvailable(CppUTest)
SET(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} ${COMPILE_FLAGS_WARNINGS}")
set(CMAKE_CXX_FLAGS_DEBUG "${COMPILE_FLAGS_DEBUG}")
set(CMAKE_CXX_FLAGS_RELEASE "${COMPILE_FLAGS_RELEASE}")
enable_testing()
### add tests
add_subdirectory(path/to/my/tests)

Answer 1

Do not use set(CMAKE_<LANG>*. Use *target* interfaces. If you do not want all stuff to be compiled with a specific option, do not set them globally. Instead:
set_target_properties(mytests PROPERTY
CXX_STANDARD 11
etc.
)
target_compile_options(mytests PUBLIC -option1 -option2 ...)
Anyway, SET(CMAKE_<LANG>_* affect targets declared after it. So move set(CMAKE_* after FetchContent_MakeAvailable(CppUTest)