How do I get cmake to use my Homebrew installation of llvm exclusively? - c++

Question

My goal is to alias my cmake command so that it uses my homebrew installation of llvm exclusively, and not the suite installed by Xcode. I would like to leave the system defaults as defaults as well. I am on Catalina with Xcode and homebrew llvm up to date.
My first step is to use the homebrew clang++ and libc++, and I can do it manually
/usr/local/opt/llvm/bin/clang++ -Wall -Wextra -std=c++17 -stdlib=libc++ \
-nostdinc++ -I/usr/local/opt/llvm/include/c++/v1 \
-L/usr/local/opt/llvm/lib -Wl,-rpath,/usr/local/opt/llvm/lib \
main.cc
For completion's sake, here's my CMakeLists.txt; it's pretty barebones as most of the work I want to do doesn't belong in CMakeLists.txt and I'm just trying to compile a toy program.
cmake_minimum_required(VERSION 3.15)
project(test)
set(CMAKE_CXX_STANDARD 17)
add_executable(test
main.cc)
But I'm already stuck with cmake here. I can specify the compiler using
-DCMAKE_CXX_COMPILER="/usr/local/opt/llvm/bin/clang++ and similar for C/clang. I am using the CMAKE_PREFIX_PATH as well, but when I open my build.ninja files, I still see it referencing Xcode directories. Adding a CMAKE_IGNORE_PATH didn't seem to get it to actually ignore /Applications/Xcode.app.
I have also tried adding the flags CMAKE_CXX_STANDARD_LIBRARIES and CMAKE_CXX_STANDARD_INCLUDE_DIRECTORIES to my cmake command, but if I do this I start to get linker errors (still using the system linker and not homebrew's ld64.lld, I believe).
Here is my current working cmake command as a reference:
cmake -DCMAKE_PREFIX_PATH="/usr/local/opt/llvm" \
-DCMAKE_CXX_COMPILER="/usr/local/opt/llvm/bin/clang++" \
-DCMAKE_C_COMPILER="/usr/local/opt/llvm/bin/clang" -G Ninja ..
The specific compiler paths are still required because cmake just breezes straight to AppleClang.
I know that the path repetition is annoying, but I want a full command to work before I start modifying my .zshrc.
Here's the broken command (broken as in building the program no longer succeeds, cmake doesn't complain here):
cmake -DCMAKE_IGNORE_PATH="/Applications/Xcode.app" -DCMAKE_PREFIX_PATH="/usr/local/opt/llvm" \
-DCMAKE_CXX_COMPILER="/usr/local/opt/llvm/bin/clang++" \
-DCMAKE_C_COMPILER="/usr/local/opt/llvm/bin/clang" \
-DCMAKE_CXX_STANDARD_LIBRARIES="/usr/local/opt/llvm/lib" \
-DCMAKE_CXX_STANDARD_INCLUDE_DIRECTORIES="/usr/local/opt/llvm/include/c++/v1" \
-G Ninja ..
With the build error message:
[2/2] Linking CXX executable test
FAILED: test
: && /usr/local/opt/llvm/bin/clang++ -isysroot /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.15.sdk -Wl,-search_paths_first -Wl,-headerpad_max_install_names CMakeFiles/test.dir/main.cc.o -o test /usr/local/opt/llvm/lib && :
ld: can't map file, errno=22 file '/usr/local/opt/llvm/lib' for architecture x86_64
clang-9: error: linker command failed with exit code 1 (use -v to see invocation)
ninja: build stopped: subcommand failed.
The variable CMAKE_CXX_LINK_EXECUTABLE just gets ignored, so it's not included.
If anyone has any guidance, I would appreciate it. All links I have found about this usually override the system defaults, and I would like to avoid doing that if possible.