[CMake] `find_package(LLVM CONFIG)` may fail when the project does not use C

[lzaoral]

1. Build LLVM 14 rc1 with LLVM_ENABLE_FFI and LLVM_ENABLE_TERMINFO options enabled.
2. Create a C++ only CMake project :

cmake_minimum_required(VERSION 3.16)  # just to silence the warning when this line is missing
project(test LANGUAGES CXX)
find_package(LLVM CONFIG REQUIRED)

3. Configure this project:

$ cmake -Bbuild . -DLLVM_DIR=/root/llvm-project-14.0.0rc1.src/llvm/build/lib/cmake/llvm/
-- The CXX compiler identification is GNU 12.0.1
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Check for working CXX compiler: /usr/bin/c++ - skipped
-- Detecting CXX compile features
-- Detecting CXX compile features - done
CMake Error at /usr/share/cmake/Modules/Internal/CheckSourceCompiles.cmake:44 (message):
  check_source_compiles: C: needs to be enabled before use.
Call Stack (most recent call first):
  /usr/share/cmake/Modules/CheckCSourceCompiles.cmake:76 (cmake_check_source_compiles)
  /root/llvm-project-14.0.0rc1.src/llvm/build/lib/cmake/llvm/FindFFI.cmake:44 (check_c_source_compiles)
  /root/llvm-project-14.0.0rc1.src/llvm/build/lib/cmake/llvm/LLVMConfig.cmake:145 (find_package)
  CMakeLists.txt:3 (find_package)


-- Could NOT find FFI (missing: HAVE_FFI_CALL)
CMake Error at /usr/share/cmake/Modules/Internal/CheckSourceCompiles.cmake:44 (message):
  check_source_compiles: C: needs to be enabled before use.
Call Stack (most recent call first):
  /usr/share/cmake/Modules/CheckCSourceCompiles.cmake:76 (cmake_check_source_compiles)
  /root/llvm-project-14.0.0rc1.src/llvm/build/lib/cmake/llvm/FindTerminfo.cmake:21 (check_c_source_compiles)
  /root/llvm-project-14.0.0rc1.src/llvm/build/lib/cmake/llvm/LLVMConfig.cmake:152 (find_package)
  CMakeLists.txt:3 (find_package)


-- Could NOT find Terminfo (missing: Terminfo_LINKABLE)
-- Found ZLIB: /usr/lib64/libz.so (found version "1.2.11")
-- Found LibXml2: /usr/lib64/libxml2.so (found version "2.9.12")
-- Configuring incomplete, errors occurred!
See also "/root/test/build/CMakeFiles/CMakeOutput.log".

Changing the project(test LANGUAGES CXX) line to project(test LANGUAGES C CXX) makes the error go away, but that's just a work-around than a solution.

CMake version: 3.22.2
Introduced by: https://reviews.llvm.org/D114327

[lzaoral]

Ping.

[sjoubert]

+1 For now this completely blocks me to update to LLVM 14 as I really don't want to enable the C language in my all C++ project.

I suppose a solution could be for the FindFFI and FindTerminfo modules to fallback to check_cxx_source_compiles if the C language is not enabled or something like that.