How to resolve linker path error when using home-brew clang 13 in CMake project on macOS 12 - macos

Question

In order to make use of newer C++ 17 and 20 features, I want to switch from Apple clang to a homebrew installed vanilla clang. I installed it via brew install llvm.
When trying to build a minimal CMake based test project like
cmake -DCMAKE_BUILD_TYPE=Debug -DCMAKE_C_COMPILER=/usr/local/opt/llvm/bin/clang -DCMAKE_CXX_COMPILER=/usr/local/opt/llvm/bin/clang++ -G Ninja testProject
CMake fails with
-- The C compiler identification is Clang 13.0.0
-- The CXX compiler identification is Clang 13.0.0
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - failed
-- Check for working C compiler: /usr/local/opt/llvm/bin/clang
-- Check for working C compiler: /usr/local/opt/llvm/bin/clang - broken
CMake Error at /Applications/CLion.app/Contents/bin/cmake/mac/share/cmake-3.20/Modules/CMakeTestCCompiler.cmake:66 (message):
The C compiler
"/usr/local/opt/llvm/bin/clang"
is not able to compile a simple test program.
It fails with the following output:
Change Dir: /Users/me/Projects/testProject/build/CLion/Debug/CMakeFiles/CMakeTmp
Run Build Command(s):/usr/local/bin/ninja cmTC_d2d4b && [1/2] Building C object CMakeFiles/cmTC_d2d4b.dir/testCCompiler.c.o
[2/2] Linking C executable cmTC_d2d4b
FAILED: cmTC_d2d4b
: && /usr/local/opt/llvm/bin/clang -isysroot /Library/Developer/CommandLineTools/SDKs/MacOSX11.0.sdk -Wl,-search_paths_first -Wl,-headerpad_max_install_names CMakeFiles/cmTC_d2d4b.dir/testCCompiler.c.o -o cmTC_d2d4b && :
ld: library not found for -lSystem
clang-13: error: linker command failed with exit code 1 (use -v to see invocation)
ninja: build stopped: subcommand failed.
So it seems that it fails to locate the correct library search path out of the box.
When trying to build a simple hello world program from the command line like
/usr/local/opt/llvm/bin/clang++ main.cpp -isysroot /Library/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX11.0.sdk
It fails with even not finding some headers.
Supplying the header and library search path explicitly to the build command like
/usr/local/opt/llvm/bin/clang++ main.cpp -I"/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include" -L"/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/lib"
makes it work. However, both are no solutions to make the built-in CMake compiler check work and they don't seem like the correct way to go.
Once this is working correctly, I'm furthermore interested in being able to compile ObjectiveC++ code which currently fails with even more errors.
I think I understand the problem that the compiler does not know the correct search paths, but I'm lacking some understanding about how it would normally figure out those search paths in a CMake project.

Answer 1

I finally managed to get it working by supplying a few linker flags through the LDFLAGS environment variable:
-L/Library/Developer/CommandLineTools/SDKs/MacOSX11.0.sdk/usr/lib
In order to find the system libraries needed by the C compiler check
-L/usr/local/opt/llvm/lib
In order to find the C++ libraries as needed by the C++ compiler check
In order to successfully compile Objective-C code that links to Apple frameworks, I furthermore needed to add
-F/Library/Developer/CommandLineTools/SDKs/MacOSX11.0.sdk/System/Library/Frameworks -lobjc
With those flags, I now can successfully build my projects. Still, I'm not 100% sure if this is the right way, especially if the libraries linked from /usr/local/opt/llvm/lib can be expected to be present on all target systems?