Why is CMake checking for the C++ compiler?

Question

I would like to build a minimal example to build a C program. I used this CMakeLists.txt file:

cmake_minimum_required(VERSION 3.6)

set(CMAKE_SYSTEM_NAME Generic)
set(CMAKE_SYSTEM_PROCESSOR arm)

set(CMAKE_C_COMPILER "arm-none-eabi-gcc")
#set(CMAKE_CXX_COMPILER "arm-none-eabi-g++")

set(CMAKE_EXE_LINKER_FLAGS "--specs=nosys.specs" CACHE INTERNAL "")

set(CMAKE_FIND_ROOT_PATH_MODE_PROGRAM NEVER)
set(CMAKE_FIND_ROOT_PATH_MODE_LIBRARY ONLY)
set(CMAKE_FIND_ROOT_PATH_MODE_INCLUDE ONLY)
set(CMAKE_FIND_ROOT_PATH_MODE_PACKAGE ONLY)

project("funambule")

list(
    APPEND src
        main.c
)

add_executable(
    funambule
    ${src}
)

When I run cmake .. CMake absolutely want to check the C++ compiler even though I don't need one. How can I prevent it to do this useless check?

-- The C compiler identification is GNU 5.4.1
-- The CXX compiler identification is GNU 5.4.0
-- Check for working C compiler: /cygdrive/c/Users/NoOne/Home/bin/arm-none-eabi-gcc
-- Check for working C compiler: /cygdrive/c/Users/NoOne/Home/bin/arm-none-eabi-gcc -- works
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - failed
-- Detecting C compile features
-- Detecting C compile features - done
-- Check for working CXX compiler: /usr/bin/CC
-- Check for working CXX compiler: /usr/bin/CC -- broken
...

Answer 1

This is the default behaviour of CMake. To change it, you should specify language for your project using:

project(<PROJECT-NAME> [LANGUAGES])

From CMake documentation:

Optionally you can specify which languages your project supports. Example languages are C, CXX (i.e. C++), Fortran, etc. By default C and CXX are enabled if no language options are given. Specify language NONE, or use the LANGUAGES keyword and list no languages, to skip enabling any languages.