[llvm] Remember LLVM_ENABLE_LIBCXX setting in installed configuration (PR #134990)

[Jan Patrick Lehr via llvm-commits]

jplehr wrote:

Hmm.. I just looked at the CMake config we use on that failing bot. I don't think we have libcxx installed on the machine and we build with GCC.

The CMake command does `-DLIBCXX_INSTALL_LIBRARY=OFF -DLIBCXX_INSTALL_HEADERS=OFF`. Not sure if this is helpful / related / the issue.

https://github.com/llvm/llvm-project/pull/134990