[llvm] Remember LLVM_ENABLE_LIBCXX setting in installed configuration (PR #139712)

[Paul Osmialowski via llvm-commits]

pawosm-arm wrote:

@Meinersbur we're trying to build it with this script: https://github.com/arm/arm-toolchain/blob/arm-software/arm-software/linux/build.sh yet after my failed attempt to use this PR, @DavidTruby has pointed me at some issue with this scipt: building libc++ in a separate step is not a recommended way, it should be built at all steps and the separate step should be dropped. I'll try that later.

https://github.com/llvm/llvm-project/pull/139712