How to compile C++ files under compiler-rt with -stdlib=libc++?

Hmm. There should be a file like llvm/runtimes/runtimes-bins/CMakeCache.txt in your build directory (might not be at that exact path, but searching for files named CMakeCache.txt in your build directory should find the one I’m after). Does that include the COMPILER_RT_USE_LIBCXX entry?