The documentation  has an example
$ cmake -G Ninja -S runtimes -B build \
-DLLVM_ENABLE_RUNTIMES="libcxx" \ -DLIBCXX_CXX_ABI=libstdc++ \ -DLIBCXX_CXX_ABI_INCLUDE_PATHS="/usr/include/c++/4.7/;/usr/include/c++/4.7/x86_64-linux-gnu/"
Unfortunately the LIBCXX_ cmake arguments aren't forwarded to the CMake
for libc++. This was discovered while working on enabling modules in the
libc++ bootstrap CI build.
Depends on D158860