[clang] [libc] [libcxx] [libc][libcxx] Support for building libc++ against LLVM libc (PR #99287)

[Louis Dionne via cfe-commits]

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@@ -293,6 +293,7 @@ option(LIBCXX_ENABLE_THREADS "Build libc++ with support for threads." ON)
 option(LIBCXX_ENABLE_MONOTONIC_CLOCK
   "Build libc++ with support for a monotonic clock.
    This option may only be set to OFF when LIBCXX_ENABLE_THREADS=OFF." ON)
+option(LIBCXX_USE_LLVM_LIBC "Build libc++ against LLVM libc." OFF)
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ldionne wrote:

We have a solution to the same problem for selecting the ABI library: `set(LIBCXX_SUPPORTED_ABI_LIBRARIES none libcxxabi system-libcxxabi libcxxrt libstdc++ libsupc++ vcruntime)`.

There's `libcxxabi` (in-tree) and `system-libcxxabi` (whatever libc++abi is installed on the system).

https://github.com/llvm/llvm-project/pull/99287