It was reverted in
35367e06b84618e21945674aa22e7cfda1957ea4
because it broke the buildbot due to missing libc++abi headers.
https://reviews.llvm.org/D75991 improves the diagnostics, so I hope
the build log will be more informative.
set(LIBCXX_TARGET_TRIPLE "${CMAKE_C_COMPILER_TARGET}" CACHE STRING "")
set(LIBCXX_SYSROOT "${DEFAULT_SYSROOT}" CACHE STRING "")
set(LIBCXX_ENABLE_SHARED OFF CACHE BOOL "")
+set(LIBCXX_CXX_ABI "libcxxabi" CACHE STRING "")
+set(LIBCXX_CXX_ABI_INCLUDE_PATHS "${CMAKE_SOURCE_DIR}/../libcxxabi/include" CACHE PATH "")
+set(LIBCXX_CXX_ABI_LIBRARY_PATH "${CMAKE_BINARY_DIR}/lib/${CMAKE_C_COMPILER_TARGET}/c++" CACHE PATH "")
set(BUILTINS_CMAKE_ARGS "-DCMAKE_SYSTEM_NAME=Linux;-DCMAKE_AR=${CMAKE_AR}" CACHE STRING "")
set(RUNTIMES_CMAKE_ARGS "-DCMAKE_SYSTEM_NAME=Linux;-DCMAKE_AR=${CMAKE_AR}" CACHE STRING "")