Revert "[libc++] Take 2: Do not cleverly link against libc++abi just because it happens to be there"

r365326 still breaks bots:
http://lab.llvm.org:8011/builders/netbsd-amd64/builds/20712/steps/ninja%20build%20local/logs/stdio
http://lab.llvm.org:8011/builders/sanitizer-x86_64-linux-autoconf/builds/39477/steps/test%20tsan%20in%20debug%20compiler-rt%20build/logs/stdio

And probably others

This reverts commit 945b9ec0693390ef35fe8c6b774495312246b8b6.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@365359 91177308-0d34-0410-b5e6-96231b3b80d8

CMakeLists.txt

@@ -149,9 +149,22 @@ set_property(CACHE LIBCXX_CXX_ABI PROPERTY STRINGS ;${CXXABIS})
 
 # Setup the default options if LIBCXX_CXX_ABI is not specified.
 if (LIBCXX_CXX_ABI STREQUAL "default")
+  find_path(
+    LIBCXX_LIBCXXABI_INCLUDES_INTERNAL cxxabi.h
+    PATHS ${LLVM_MAIN_SRC_DIR}/projects/libcxxabi/include
+          ${LLVM_MAIN_SRC_DIR}/runtimes/libcxxabi/include
+          ${LLVM_MAIN_SRC_DIR}/../libcxxabi/include
+    NO_DEFAULT_PATH
+    NO_CMAKE_FIND_ROOT_PATH
+  )
   if (LIBCXX_TARGETING_MSVC)
     # FIXME: Figure out how to configure the ABI library on Windows.
     set(LIBCXX_CXX_ABI_LIBNAME "vcruntime")
+  elseif ((NOT LIBCXX_STANDALONE_BUILD OR HAVE_LIBCXXABI) AND
+          IS_DIRECTORY "${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)
   elseif (APPLE)
     set(LIBCXX_CXX_ABI_LIBNAME "libcxxabi")
     set(LIBCXX_CXX_ABI_SYSTEM 1)

cmake/Modules/HandleLibCXXABI.cmake

@@ -98,7 +98,7 @@ if ("${LIBCXX_CXX_ABI_LIBNAME}" STREQUAL "libstdc++" OR
     "${_LIBSUPCXX_LIBNAME}" "${_LIBSUPCXX_LIBNAME}" "${_LIBSUPCXX_INCLUDE_FILES}" "bits"
     )
 elseif ("${LIBCXX_CXX_ABI_LIBNAME}" STREQUAL "libcxxabi")
-  if (TARGET cxxabi_shared AND TARGET cxxabi_static)
+  if (LIBCXX_CXX_ABI_INTREE)
     # Link against just-built "cxxabi" target.
     set(CXXABI_SHARED_LIBNAME cxxabi_shared)
     set(CXXABI_STATIC_LIBNAME cxxabi_static)