Reland "[CMake][libcxx] Check that we have libcxxabi before using it"

This relands commit r291726.

llvm-svn: 292086
Cr-Mirrored-From: sso://chromium.googlesource.com/_direct/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: 2203aa9a280037136ccca64ac5620a6ceba11f8e
diff --git a/CMakeLists.txt b/CMakeLists.txt
index b7d2f4a..8174e75 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -125,7 +125,8 @@
           ${LLVM_MAIN_SRC_DIR}/runtimes/libcxxabi/include
     NO_DEFAULT_PATH
   )
-  if (IS_DIRECTORY "${LIBCXX_LIBCXXABI_INCLUDES_INTERNAL}")
+  if ((NOT LIBCXX_STANDALONE_BUILD OR HAVE_LIBCXXABI) AND
+      IS_DIRECTORY "${LIBCXX_LIBCXXABI_INCLUDES_INTERNAL}")
     set(LIBCXX_CXX_ABI_LIBNAME "libcxxabi")
     set(LIBCXX_CXX_ABI_INCLUDE_PATHS "${LIBCXX_LIBCXXABI_INCLUDES_INTERNAL}")
     set(LIBCXX_CXX_ABI_INTREE 1)