b010b7ea89
Updated version of #134990 which was reverted because of the buildbot [openmp-offload-amdgpu-runtime-2](https://lab.llvm.org/buildbot/#/builders/10) failing. Its configuration has `LLVM_ENABLE_LIBCXX=ON` set although it does not even have libc++ installed. In addition to #134990, this PR adds a check whether C++ libraries are actually available. `#include <chrono>` was chosen because this is the library that [openmp-offload-amdgpu-runtime-2 failed with](https://github.com/llvm/llvm-project/pull/134990#issuecomment-2792138162). Original summary: The buidbot [flang-aarch64-libcxx](https://lab.llvm.org/buildbot/#/builders/89) is currently failing with an ABI issue. The suspected reason is that LLVMSupport.a is built using libc++, but the unittests are using the default C++ standard library, libstdc++ in this case. This predefined `llvm_gtest` target uses the LLVMSupport from `find_package(LLVM)`, which finds the libc++-built LLVMSupport. To fix, store the `LLVM_ENABLE_LIBCXX` setting in the LLVMConfig.cmake such that everything that links to LLVM libraries use the same standard library. In this case discussed in https://github.com/llvm/llvm-zorg/pull/387 it was the flang-rt unittests, but other runtimes with GTest unittests should have the same issue (e.g. offload), and any external project that uses `find_package(LLVM)`. This patch fixed the problem for me locally.
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